To show the effectiveness of a brief group alcohol intervention. Aims of the intervention were to reduce the frequency of heavy drinking occasions, maximum number of drinks on an occasion and overall weekly consumption. A cluster quasi-randomized control trial (intervention n = 338; control n = 330) among 16- to 18-year-old secondary school students in the Swiss Canton of Zürich. Groups homogeneous for heavy drinking occasions (5+/4+ drinks for men/women) consisted of those having medium risk (3–4) or high risk (5+) occasions in the past 30 days. Groups of 8–10 individuals received two 45-min sessions based on motivational interviewing techniques. Borderline significant beneficial effects (p < 0.10) on heavy drinking occasions and alcohol volume were found 6 months later for the medium-risk group only, but not for the high-risk group. None of the effects remained significant after Bonferroni corrections. Group intervention was ineffective for all at-risk users. The heaviest drinkers may need more intensive treatment. Alternative explanations were iatrogenic effects among the heaviest drinkers, assessment reactivity, or reduction of social desirability bias at follow-up through peer feedback.