The emergence of Internet technology provides a unique opportunity to collect real-time information on the sexual behaviors of persons at risk for HIV. However, relatively little is known of the utility and reliability of web diaries. To address this gap, 45 predominantly Caucasian and gay, Internet-using men who have sex with men completed a 4-week daily web diary of their sexual behaviors. Afterward, participants completed an online retrospective recall survey (RRS) of their sexual activities during the diary period and satisfaction ratings of the web diary method. Overall, web diary estimates of sexual behaviors differed from the RRS, with a tendency to over-report on the latter with the exception of insertive unprotected anal intercourse. Additionally, compliance rates were high and there was some evidence for reactivity. Participants evaluated the web diary method positively. Web diaries appear to be a promising tool for collecting information on health behaviors.