Young people's sexual health is of growing concern. Past studies have been limited by their focus on the individual. More useful are interaction-orientated approaches that focus on partners and on the social context, rather than on the individual alone. This paper describes a study of 8068 men in Mexico City that takes this new approach. Multivariate techniques were used to examine use of contraception for pregnancy prevention and for sexually transmitted disease prophylaxis among unmarried, heterosexually active men aged 15-24. Friends and sexual partners were found to play an important role in shaping risk behaviour. To explain such behaviours adequately we need to take account of more than the characteristics of individuals isolated from their social contexts. © 2004 Taylor & Francis Ltd.