Data from a cross-sectional survey of the health of Ontario children carried out in 1983 were used to provide estimates of the prevalence, patterns and sociodemographic correlates of the use of tobacco, alcohol and illicit drugs (substance use) among adolescents aged 12 to 16 years. Ninety-one percent of selected households participated. The prevalence rates of all categories of substance use, except use of inhalants, increased with increasing age. Among children aged 14 to 16 years the rates for girls were higher than those for boys for all categories of substance use except use of other, nondefined drugs. The prevalence rates of substance use tended to be higher in small urban areas except for use of marijuana (more prevalent in large urban areas) and use of inhalants (more prevalent in rural areas). The strongest evidence of clustering of substance use within families was found for smoking. Children who used less prevalent drugs (e.g., "hard" drugs) also tended to use the more prevalent ones (e.g., marijuana, tobacco and alcohol). Associations between substance use and low socioeconomic status suggested a positive relation with smoking and a negative relation with use of alcohol. The findings highlight the need for preventive programs aimed at specific subgroups in the adolescent population.