This study examined the agreement between daily and retrospective reports of vaginal sex over a two-month period in a sample of 376 heterosexually active men and women. We also examined whether gender, age, or method of daily data collection (self-administered vs. interviewer administered) were related to agreement between daily and retrospective reports. Both counts and categorical measures of frequency of the behaviors were examined. There were no gender, age, or data collection method effects. When measured as a count, participants reported more instances of vaginal intercourse in the retrospective reports than on the daily reports. In contrast, comparison of retrospective categorical measures of frequency to daily reports showed considerable variability. Possible reasons for the over-reporting of counts of vaginal sex in retrospective reports are explored.