In a study of the accuracy of self-reports of sexual behavior and condom use, 285 single, young adults in a large metropolitan area were interviewed once a week for 52 weeks, reporting on their sexual behavior each week. At 1-, 3-, 6-, and 12-month intervals, the individuals also provided self-reports of the number of times they had engaged in sex and the number of times they had used condoms since they began participating in the study. These self-reports were compared with aggregates of the weekly data, which served as a comparison standard for accuracy evaluation. The results indicate a fairly high level of accuracy in self-reports, but with 2% to 5% outliers who are highly inaccurate. The results tend to favor the assessment of sexual behavior over moderate time durations (3 or 6 months) rather than short or long durations (I month or 12 months) when trying to maximize self-report accuracy. Accuracy was attenuated for individuals who tended to engage in sex frequently, especially at the 12-month time durations.