AIMS:To investigate how gender composition of the drinking group affects young adults' alcohol consumption on weekend evenings over and above the effect of drinking-group size. DESIGN:Using the internet-based cellphone-optimized assessment technique (ICAT), participants completed online questionnaires on their cell phones every hour from 8 p.m. to midnight on Thursday, Friday and Saturday evenings during five consecutive weekends. SETTING:French-speaking Switzerland. PARTICIPANTS:Convenience sample of 183 young adults (53.0% female, mean age = 23.1) who completed a total of 4141 hourly assessments. MEASUREMENTS:Alcohol consumption and number of male and female friends present assessed at 8 p.m., 9 p.m., 10 p.m., 11 p.m. and midnight. FINDINGS:Results of three-level negative binomial regression analyses showed that women consumed significantly more drinks per hour when drinking in mixed-gender groups (Z-values ranging from 2.9 to 5.3, all P < 0.01) and significantly fewer drinks when drinking with men only (Z = -2.7, P < 0.01), compared with drinking with women only. Men reported consuming more drinks per hour in mixed-gender groups of equal gender composition (Z = 2.4, P < 0.05) or mixed-gender groups with men in the majority (Z = 2.2, P < 0.05) and fewer hourly drinks when drinking with women only (Z = -4.9, P < 0.001), compared with drinking with men only. Drinking-group size predicted the hourly number of drinks for women (Z = 6.0, P < 0.001) and men (Z = 5.5, P < 0.001). CONCLUSIONS:Drinking-group gender composition is associated with number of drinks consumed per hour, over and above the impact of the drinking-group size. Young adults report consuming more drinks per hour when drinking with mixed-gender groups than with same-gender groups.