The purpose of this study was to determine the prevalence and time trend of cigarette, alcohol, and illicit substance use among adolescent students in Taiwan, from 1991 through 1996. Subjects were selected through a two-stage random-sampling procedure. In the first stage, the strata were based on the type of school: middle school (13-15 years), high school (16-18), vocational school (16-18), and junior college (first 3 years only, 16-18). In the second stage, the strata were based on grade: one class each for 1st, 2nd, and 3rd grades were randomly selected from each type of school. Students were guaranteed anonymity before the questionnaire was administered. About 12,000 students from 100 schools participated each year in 1991 and 1994-1996. In 1993, 8,320 students from 65 schools participated. The prevalence of smoking and drinking decreased between 1991 and 1994, and then increased between 1994 and 1996. In 1996, the prevalence of illicit substance use was 15.4% for smoking, 16.7% for drinking, and 1.5% for illicit substance use. The rate of illicit substance use was lowest among high school students, followed by middle school students, and higher among students in vocational schools and junior colleges. The percentage of smokers and drinkers who had started early (at age 12 or earlier) rose every year. The prevalence of smoking and drinking habits among girls increased consistently during the study period. The most commonly abused substance was amphetamine. However, in 1996, sniffing glue became more prominent among middle school students, and flunitrazepam became the second most commonly abused substance among high school students.