This paper describes heterosexual behaviour, and protection against pregnancy and sexually transmitted diseases, among 908 18-year-olds in the Glasgow area in 1990. The proportion who reported having experienced sexual intercourse was similar to, though slightly lower than, that reported in most recent studies elsewhere. There were marked sex differences, with males reporting being significantly younger at first intercourse and having had more sexual partners than females. The reported prevalence of unprotected sex (without any contraception) and of "unsafe" sex (without a condom) was high. Being male, of no religious affiliation, an early school leaver, and not in higher education were independently associated with sexual experience. Among the sexually experienced, lower education was also associated with "unsafe" sex, and Catholicism with unprotected sex. The overall pattern of findings, particularly in relation to sex differences, suggests that young people in the Glasgow area may be subject to more traditional influences than those elsewhere.