To determine the prevalence of high-risk sexual behaviours and the influence of substance use and unplanned sexual intercourse on multiple sexual partners, inconsistent condom use and reasons for not always using condoms among adolescent students. A standardized self-reported anonymous questionnaire administered to a representative sample of students. The Canadian provinces of Nova Scotia, New Brunswick, Newfoundland and Labrador, and Prince Edward Island in 1998. 9997 students in grades 9, 10 and 12 in the public school system. Items on sexual intercourse, unplanned sexual intercourse, number of sexual partners, condom use, alcohol use, episodes of binge drinking and drunkenness, cigarette smoking and cannabis use. About 37.5% of males and 39.7% of females reported having engaged in sexual intercourse in the 12 months prior to the survey. Of those, 68.0% of males and 61.5% of females reported having engaged in unplanned sexual intercourse, 40.9% of males and 32.1% of females reported having more than one sexual partner, and 49.9% of males and 64.1% of females reported inconsistent condom use. Unplanned sexual intercourse under the influence of alcohol or other drug was found to be an independent risk factor for multiple sexual partners and inconsistent condom use. The demonstration of an association between substance use, unplanned sexual intercourse and other sexual behaviours lends support to a harm minimization approach, including the provision of non-judgemental information and interventions addressing unplanned sexual intercourse under the influence of a substance.