To study the association of the consumption of alcohol and other psychoactive drugs with sexual risk behaviour for HIV infection, data from a representative sample of the Spanish population aged 18-39 years were analysed. A national household survey was carried out in 1996 using a combination of face-to-face interviews and self-administered questionnaires. The survey included 5253 subjects aged 18-39 years who provided information on alcohol and drug consumption, number of sexual partners and condom use with the steady partner and with casual partners in the 12 months before the survey. Of those surveyed, 27.4% had been drunk at least once and 20.5% had consumed drugs. Both behaviours were associated with male sex, younger age, higher educational level, being single and having had more than one sexual partner. In the logistic regression analysis adjusting for the sociodemographic variables, the greater frequency of drunkenness and cannabis use were associated with having more than one sexual partner. Regular condom use was significantly less frequent among cocaine users and more frequent among opiate users, but was not associated with the use of other drugs. Sexual risk behaviour (i.e. more than one partner and failure to use a condom regularly) was more frequent among persons who had been drunk or used cannabis or cocaine. Excessive consumption of alcohol, and cannabis and cocaine use are independently associated with sexual behaviour involving greater risk of HIV infection or transmission.