There is a question that has crossed the mind recently of anyone who has sent a cellphone text message while cheating on a spouse: What was I thinking?

Text messages are the new lipstick on the collar, the mislaid credit card bill. Instantaneous and seemingly casual, they can be confirmation of a clandestine affair, a record of the not-so-discreet who sometimes forget that everything digital leaves a footprint.

This became painfully obvious a week ago when a woman who claims to have had an affair with Tiger Woods told a celebrity publication that he had sent her flirty text messages, some of which were published. It follows on the heels of politicians who ran afoul of text I.Q., including a former Detroit mayor who went to prison after his steamy text messages to an aide were revealed, and Senator John Ensign of Nevada, whose affair with a former employee was confirmed by an incriminating text message.

Unlike earlier eras when a dalliance might be suspected but not confirmed, nowadays text messages provide proof. Divorce lawyers say they have seen an increase in cases in the past year where a wronged spouse has offered text messages to show that a partner has strayed. The American Bar Association began offering seminars this fall for marital attorneys on how to use electronic evidence text messages, browsing history and social networks in proving a case.

How does someone make up an excuse when what is happening is right there, written in black and white? asked Mitchell Karpf, a Miami divorce lawyer who is also chairman of the bar association's family law section. By the time someone shows up with a handful of texts, there is no going back.

Although most e-mail users have come to understand that messages remain on their computers even if deleted, text messages are often regarded as more ephemeral type, hit "send" and off it goes into the ether. But messages can remain on the sender's and receiver's phones, and even if they are deleted, communications companies store them for anywhere from days to a few weeks. AT&T said that, at most, it saved text messages for 72 hours while Verizon said it saved them for 5 to 10 days.

Lawyers expect the number of cases to grow as younger cellphone users, who are more likely to text than talk, marry. Text messages now outnumber mobile voice calls three to one, according to the Nielsen Company. Monthly messages sent or received jumped to 584 a person in the quarter ending in September, a 60 percent increase from a year earlier.

At the root of the issue is privacy or rather the increasing lack of it in our show-and-tell digital culture. Text messages are considered private, much as telephone calls are, legal experts say. But if a cheating spouse's cellphone is part of a family calling plan or regularly left unlocked and unattended on the dinner table or night stand, it is conceivable that a partner who suspects infidelity could make a case for sifting through the in-box.

People who have something really private to say probably shouldn't do it in a text on their cellphone, said Marc Rotenberg, executive director of the Electronic Privacy Information Center, a public interest research group based in Washington.

In Mr. Woods's case, Jaimee Grubbs, who has worked as a cocktail waitress, came forward with text messages that she said were from Mr. Woods once he was rumored to be having marital problems after he slammed his car into a fire hydrant and a tree on Thanksgiving. Since then, several other women have said they, too, slept with Mr. Woods. He has said in a statement only that he was sorry for his "transgressions" and asked that his family be left alone.

Personal sins should not require press releases, and problems within a family shouldn't have to mean public confessions, Mr. Woods said.

Others, like Kwame Kilpatrick, the former mayor of Detroit, were found out because they used government-issued mobile phones and pagers. Mr. Kilpatrick lied under oath about having an affair with an aide, but his text messages revealed the truth. Nevada's governor, Jim Gibbons, was accused last spring by his wife in divorce documents of sending more than 800 text messages to a mistress in 2007. He contended that the woman was a friend, but he paid the state $130 for the messages from his phone.

What is more common, though, is suspicion followed up by a confrontation. Doug Hampton, a longtime friend and employee of Senator Ensign's, said recently on the ABC show "Nightline" that he was alarmed after he had borrowed Mr. Ensign's cellphone in late 2007 to call his wife, Cynthia Hampton, and found her listed as "Aunt Judy." Mr. Hampton said he found an incriminating text message and confronted the pair about their affair at a Christmas dinner soon after.

In a recent survey of 2,300 adults about social networking, the Pew Internet and American Life Project found that 12 percent said they had shared information online that they later regretted posting. Posting on a social network is not the same as sending a text message. But Lee Rainie, director of the Pew project, contends it is evidence of an overall cultural shift in which people have become increasingly careless about revealing personal information they cannot take back.

It is one thing to write a personal note to someone who shares it with her two best friends, said Mr. Rainie. It is another thing to text your undying affection and become a laughingstock. What feels intimate and anonymous at the time, perhaps, really isn't. It can be shared widely.

Sherry Turkle, a professor and researcher at the Massachusetts Institute of Technology, has studied interaction with technology for more than two decades. Unlike with computers, Professor Turkle said, consumers have a deeply personal connection to their cellphones, where they keep contact lists and family photos. They carry them in their pockets, she said, next to their skin.

One woman Professor Turkle spoke to for a study was so grief-stricken after she had misplaced her cellphone that she described the loss as a death. "People feel it is an extension of their body and mind," the professor said, but, she added: "Like Peter Pan, we do not see our electronic shadow until it is pointed out to us. We assume it is not there."

Proving adultery is not the only value of a text message to a divorce lawyer. Last year Mr. Karpf, the lawyer from Miami, represented a husband whose wife was seeking sole custody of their child. The wife claimed the husband had left her and the child. He countered, saying he left because she was physically abusive. She denied it until Mr. Karpf produced several text messages the wife sent her husband apologizing for her inappropriate behavior. "She set up the whole case for me," Mr. Karpf said.

Robert Stephan Cohen, the lawyer who represented Christie Brinkley in her divorce from Peter Cook, said a spouse's finding out about a cheating partner by reading their personal text messages would have a profound effect on how such cases were played out, both in court and among friends and family. Mr. Cohen predicted that the battles in even the most routine divorces would become uglier with more text messages as evidence.

"It's much different than rumor running around about a husband at dinner with a babe in the back booth," he said. It's in the spouse's face. They read it over and over again. It's harsh and hurtful.