PMSbuddy, a Web site launched in 2008 by Vanderbilt University alum Jordan Eisenberg, 27, takes the mystery out of the mood swings. The site alerts selected loved ones when a woman is likely to experience PMS. There's even an iPhone app for that. The site also includes a forum for discussion and a tips page.

The 100,000-plus members, 85 percent of whom are men in relationships, engage in discussions ranging from asking for advice on how to handle PMS to tips for cures. Many of the men, Mr. Eisenberg said, were registered by their wives or girlfriends.

The site, he said, was born out of a conversation among a group of friends. A friend had begun tracking his wife's cycle on a Palm Pilot in order to avoid asking indelicate questions.

"I wondered if it was something people could really use," said Mr. Eisenberg, who added that the site is intended to be taken with a grain of amused salt.

"The name is PMSbuddy," he said. "It's not serious. It's not medical advice. It's certainly tongue-in-cheek, however, we have thousands of users who find it incredibly helpful, both men and women. If it allows the man in the relationship to be more considerate and understanding, we think it's great."

But not everyone is laughing.

"I'm appalled if anybody is seriously doing this because I do think it would contribute to the notion that PMS is a widespread affliction among women, that they're irrational," said Marcia Noe, head of the Women's Studies department at the University of Tennessee at Chattanooga.