This study examines issues in the measurement of HIV risk factors, using daily diaries to collect data on both alcohol use and sexual behavior. Seventy-nine adolescents and young adults recruited from an STD clinic and from a university campus were studied. Participants gave daily reports of their drinking, drug use, and sexual activities for 4 weeks. Respondents then completed a retrospective questionnaire asking about the frequencies of these behaviors during the preceding period. Diary reports of behavior were strongly correlated with retrospective reports. More frequent drinking was reported on the diary measure than the retrospective measure, and this discrepancy was larger for more frequent drinkers. Frequency of sexual activity was overreported on the retrospective measure only among adolescents. Errors in the measurement of alcohol use, sexual behavior, or their co-occurrence could affect estimations of the relationship of alcohol use to sexual behavior. The types of error inherent in these measures may differ, resulting in different tendencies toward over- or underreporting of alcohol use and sexual behavior, depending on frequency of the behavior and the characteristics of the subject population.