ABSTRACT. Objective: This research examined the relationship between the intragroup familiarity among peers visiting nightclubs and the likelihood of experiencing negative outcomes. We hypothesized, based on our prior work, that members who are more familiar with their group would be more likely to experience sexual and physical aggression while at the club. Method: The study involved 1,765 young adults (within 654 natural groups) sampled at nightclubs in the San Francisco Bay area. Participants were interviewed about their clubbing history and expectations before entering the club and about their experiences in the club as they exited. Breath samples were collected at both entry and exit to obtain objective measures of alcohol use. Results: Using generalized linear mixed modeling to accommodate correlated data, we found that, to the extent that club patrons were familiar with more of their peer group, the more likely they were to experience sexual and physical aggression, although this was moderated by participant gender. Conclusions: Although in many circumstances group cohesion can be a protective factor, the results of this study suggest that greater group familiarity might sometimes be associated with less concern for safety, reduced vigilance, and an increase in negative experiences. (J. Stud. Alcohol Drugs, 76, 924-927, 2015)