AIM: To test the efficacy of an internet-based brief intervention (IBI) in decreasing alcohol use among young Swiss men aged 21 years on average. DESIGN: Two parallel-group randomized controlled trial with a 1 : 1 allocation ratio containing follow-up assessments at 1 and 6 months post-randomization SETTING: Internet-based study in a general population sample. PARTICIPANTS: Twenty-one-year-old men from Switzerland with unhealthy alcohol use (> 14 drinks/week or ≥ 6 drinks/occasion at least monthly or Alcohol Use Disorders Identification Test (AUDIT) scores ≥ 8) INTERVENTION: IBI consisting of (1) normative feedback, (2) feedback on consequences of alcohol use, (3) calorific value of reported consumption, (4) computed blood alcohol concentration for reported consumption, (5) indication of risk, (6) information on alcohol and health and (7) recommendations indicating low-risk drinking limits. Control condition: no intervention (assessment only). MEASUREMENTS: At 1 and 6 months: quantity/frequency questions on alcohol use (primary outcome: number of drinks/week) and binge drinking prevalence; at 6 months: AUDIT score, consequences of drinking (range = 0-12). FINDINGS: Follow-up rates were 92% at 1 month and 91% at 6 months. At 6 months, participants in the intervention group (n = 367) reported greater reductions in the number of drinks/week than participants in the control group (n = 370) [treatment × time interaction, incidence rate ratio (RR) = 0.86, 95% confidence interval (CI) = 0.78; 0.96], but no significant differences were observed on binge drinking prevalence. There was a favourable intervention effect on AUDIT scores (IRR = 0.93, 95% CI = 0.88; 0.98), but not on the number of consequences (IRR = 0.93, 95% CI = 0.84; 1.03). CONCLUSIONS: An internet-based brief intervention directed at harmful alcohol use among young men led to a reduction in self-reported alcohol consumption and AUDIT scores compared with a no-intervention control condition (assessment only).