BACKGROUND: Research in the United States and the United Kingdom indicates that drinking before going out (commonly called "predrinking") is common among young people and associated with increased harm. On the basis of Swiss data, this study investigates differences in alcohol consumption and adverse or risky outcomes for evenings when persons consumed alcohol before going to a licensed premise (i.e., predrinking), drank on-premise only, or drank off-premise only. METHODS: Using the recently developed Internet-based cell phone-optimized assessment technique (ICAT), alcohol consumption and drinking location were assessed at 6 time points (5 pm to the next morning) on Thursdays, Fridays, and Saturdays over 5 consecutive weeks by means of participants' cell phones. Overall, 7,828 assessments provided by 183 young adults (53.0% women, mean age [SD] = 23.1 [3.1]) on 1,441 evenings were analyzed by means of cluster-adjusted means and proportion tests and of multilevel structural equation models. The extent to which alcohol consumption mediated the association between predrinking and adverse outcomes was also examined. RESULTS: Higher alcohol consumption occurred on evenings with predrinking (7.1 drinks on average) compared with on-premise only (4.2 drinks) and off-premise only (4.3 drinks) evenings. Adverse outcomes occurred more often on evenings with predrinking (with 23.8% of predrinking nights involving at least 1 outcome) than on evenings with on-premise drinking only (13.9%) and off-premise drinking only (12.0%). Predrinking was indirectly associated with adverse outcomes, mediated by larger amounts of alcohol consumed in the evening. CONCLUSIONS: Because of its association with heavier consumption and related adverse outcomes, predrinking, especially combined with on-premise drinking, represents a major target for prevention. Educational interventions as well as structural measures, such as reduction in late-night off-sale opening hours, and staff training in responsible beverage service, are needed to prevent high total consumption and related adverse consequences among young people.