Perceived peer drinking, social affiliation, and gender were tested as moderators of two indexes of heavy drinking. A battery of self-report questionnaires included the Interpersonal Orientation Scale, which assessed social affiliation, and the Daily Drinking Questionnaire, which provided indexes of weekly drinking by participants and their perceptions of weekly drinking by same-gender peers. Two indexes of drinking were computed: weekend drinking (the sum of drinks consumed on Friday and Saturday) and frequency of binge drinking as defined for men and women. Data analyses found that increased drinking at college entrance reflected perceived increases of drinking by peers. Also, perceptions of peer drinking were robustly overestimated, with women showing the larger overestimation bias. Social affiliation was associated with men's drinking and moderated its relation to perceived peer drinking at college entrance. Both men and women reported marked increases in four indexes of drinking upon entering college: drinking frequency, drinking quantity, weekend drinking, and frequency of binge drinking. Perceived peer norms for drinking appear to be a target for effective intervention, given the weight of their influence and the relative ease with which they have been addressed in some campus programs to counter heavy drinking. 3 figures, 2 tables, and 25 references