Aims: The aim of this study was to evaluate the outcomes of Cool and Clean, Switzerland's largest substance use prevention programme, targeted specifically at 10- to 20-year-olds who belong to a sports club and train as part of a team. Method: Based on a representative sample of young people who belong to a sports club and train as part of a team (both participants and non-participants in "cool and clean"), a post-test-only static group comparison was used to analyse data on 1,887 individuals from 179 teams (mean age: 13.8; 33.5% girls). Outcome variables included sports-related aspects (achievement motivation, fair play); attitudes towards tobacco, cannabis and alcohol use, and doping; and substance use (tobacco, cannabis and alcohol use). Results: Cool and Clean participants self-reported beneficial effects of the programme in all outcome areas. They also had a more critical attitude towards doping and substance use in general and had a lower reported frequency of cannabis use than non-participants. We found no significant group differences in terms of sports-related variables, tobacco and alcohol use. However, in each of these areas, better outcomes were found among Cool and Clean participants who trained with teams that implemented the programme more comprehensively. Conclusions: There is cross-sectional evidence for the effectiveness of the Cool and Clean programme. Moreover, the level of impact of the programme depends not only on participation per se but also, and more decisively, on how comprehensively the programme is implemented.