The aim of this study was to assess whether coping styles, social stress, and sexual sensation seeking were predictors of HIV/STD risk behaviours in adolescents. A representative sample of 4,456 female and male Spanish high school students aged 13 to 18 years participated. A stratified random sampling procedure was used. Self-report questionnaires on coping styles, social stress, sexual sensation seeking, and sexual behavior were administered to participants at their high schools. Sexual sensation seeking predicted whether sex was with a casual partner and whether drugs were used at the last sexual intercourse in males and females. Nonproductive coping was a risk factor in the use of drugs at the last sexual intercourse in males. Problem-solving coping style was related to a lower likelihood in the use of drugs at the last sexual encounter in females and to a higher likelihood of condom use at last sexual intercourse in males. Social stress increased the likelihood of having used drugs at last sexual intercourse in females. This study highlights the need to incorporate sexual sensation seeking, social stress, and coping styles in HIV/STD prevention programs aimed at adolescents. (Contains 6 tables.)