Abstract: Practicing sexual intercourse (SI) under the influence of alcohol or drugs has been identified as an unhealthy sexual behaviour. The sample consisted of 5695 adolescents, of which 46.1% were males, with a mean age of 15 years old. The measures consisted of asking the adolescent if he or she had ever had SI, age of 1st SI, condom use at last SI, SI under the influence of alcohol or drugs and HIV testing. Most adolescents reported having never had SI (77.0%). Among those who responded affirmatively, they referred having had their 1st SI at 15 years old. A significant minority reported not having used condom at last SI (34.1%) and having had SI under the influence of alcohol or drugs (14.5%). It was noticeable that males, adolescents with high SES, those who reported not having used condom at last SI, those not having been HIV tested, those who consider themselves fat, smoke daily, and scored lower in emotional sensitivity and quality of life were those who were more likely to have reported having had SI under the influence of alcohol or drugs. Adolescents who reported having had SI under the influence of alcohol or drugs have multiple risk behaviours. These results may account for important changes in educational and health policies, directed towards the development of personal and social skills, the importance of adolescents’ quality of life and the activation of all the support structures where adolescents are involved and that are somewhat responsible for promoting a healthy lifestyle. Keywords: unhealthy sexual behaviour, sexual intercourse, substance use, Portuguese adolescents