To examine whether young adults' alcohol consumption during the course of an evening was affected by the number of friends present, and the interaction between participants' gender and number of friends present.Participants used the internet-based cellphone-optimized assessment technique (ICAT) to complete a series of cellphone questionnaires every Thursday, Friday and Saturday evening over five weekends. A multi-level growth curve model (hourly assessments, clustered within evenings, clustered within individuals) with time-invariant and time-varying covariates was estimated.French-speaking Switzerland.A total of 183 young adults (53.0% female, mean age = 23.1) who completed 7205 questionnaires on 1441 evenings.Alcohol consumption and number of friends present assessed at 8 p.m., 9 p.m., 10 p.m., 11 p.m. and midnight.Drinking pace accelerated notably over the course of the evening on Saturdays (b = 0.047; P < 0.01). Men consumed more alcohol than women, particularly at the beginning of the evening (b = 0.152; P < 0.05). However, this effect was no longer significant when the impact of friends was accounted for (b = 0.096; P = 0.139). The higher the number of friends present, the higher the number of drinks consumed at a given time during the course of the evening (b = 0.070; P < 0.001). Cross-level interaction effects indicated that this relationship was stronger for men than women (b = 0.027; P < 0.05).Among young adults in Switzerland, the number of friends present is associated positively with hourly drinking frequency during the course of weekend evenings. The impact of the drinking group size on alcohol use is stronger for men than women.