AbstractObjective: To examine birth cohort trends in the prevalence, age of initiation and durability of the use of psycho-stimulant and psycho-depressant drugs in Australia between 1988 and 1998.Method: Nine synthetic birth cohorts were constructed from a pooled dataset of the three National Drug Strategy Household Surveys (1988, 1993, 1998). The design-effect adjusted weighted lifetime and recent prevalence, age of initiation and durability of use, of three popular psycho-stimulants (amphetamine, cocaine and ecstasy) and three popular psycho-depressants (cannabis, heroin and inhalants) were estimated. Significance tests of difference use chi-square statistics and the sociodemographic predictors of trends were tested using logistic regression techniques.Results: Lifetime and recent prevalence and durability of use increased and ages of initiation decreased, with each successive cohort. Psycho-stimulants were predictors for psycho-depressant use and vice versa. Being male, aged under 35 years, Australian born, employed or in fulltime education, and a tobacco smoker were significant predictors of increased risk of using both classes of drugs. Being married or having no post-secondary education qualifications were significant predictors of lower risk of using either class of drugs.Conclusions: Younger cohorts are more likely to use both classes of psychoactive drugs, to have commenced using the drugs earlier and to have maintained their drug habits longer, compared to older cohorts. While increased availability is an important factor in the likelihood of exposure to illicit drugs, interventions implemented since the National Drug Strategy began in 1985 do not appear to have reduced the probability that young people will initiate and maintain illicit psychoactive drug use, exposing them to associated health and legal consequences.