We compared quantitative diary data with retrospective survey data collected from a cohort of young adult men who have sex with men (MSM) in Seattle, Washington. Ninety-five MSM, aged 16-30 years, completed web-based surveys every 3 months and were randomized to 4 diary submission schedules: every 2 weeks, once a week, twice a week, or never. We calculated diary completion rates and assessed agreement between daily diary data and aggregate retrospective survey data for sexual behavior measures. Over 6 months, 78 % of participants completed at least 80 % of their diary days, and the 2-week schedule had the highest and most consistent completion rate. The majority of sexual behavior and substance use measures had strong agreement between the diary and retrospective survey data (i.e., kappa >0.80 or concordance correlation coefficient ≥0.75), although we observed poorer agreement for some measures of numbers of anal sex acts. There were no significant differences in mean responses across diary schedules. We observed some evidence of reactivity (i.e., a difference in behavior associated with diary completion). Participants not assigned diaries reported significantly more unprotected anal sex acts and were more likely to be newly diagnosed with HIV or another sexually transmitted infection compared to those assigned active diary schedules. This study suggests that sexual behavior data collected from young adult MSM during 3-month retrospective survey--an interval commonly used in sexual behavior research--are likely valid. Diaries, however, may have greater utility in sexual behavioral research in which counts, timing, sequence, or within-person variation over time are of particular import.