To obtain up-to-date prevalence estimates of tobacco smoking and alcohol drinking among Australian secondary students, and to compare these estimates with those obtained from similar studies conducted in 1984 and 1987. Data were collected from 24,892 secondary students aged 12 to 17 years from all Australian States and the Northern Territory. A stratified two-stage sample design was used. First, a random sample of schools was selected and second, for each school a sample of 80 students was randomly selected from predetermined year levels. A total of 351 schools participated in the survey. Students completed an anonymous, self-administered questionnaire on their smoking and drinking behaviours. The prevalence of current smoking (defined as having smoked at least one cigarette in the week preceding the survey) was found to increase with age to reach a peak of 25% among 16-year-old boys and 29% among girls aged 15 years. From the age of 13, smoking was more prevalent among girls than boys. Among current smokers, boys were heavier smokers than were girls. Unlike smoking, drinking was slightly more prevalent among boys than girls; boys were heavier drinkers. The proportion consuming at least one alcoholic drink in the week before the survey rose with age to a peak of 51% of boys and 46% of girls aged 17 years. Comparisons with data obtained from a similar survey conducted in 1987 showed that there had been a decrease in the proportion of 12 to 15 year olds smoking. The prevalence of drinking among both 12 to 15 year olds and 16 to 17 year olds was significantly lower in 1990 than 1987. While the continuing downward trends in smoking and drinking among younger students is encouraging, the results show that there are still large numbers of students smoking and drinking.