['Relatively little is known about why almost half of young adults in Australia have used cannabis. Because the upwards trend in use of cannabis has been coincident with an increase in marital breakdown, this study examines the relationship between marital status, marital changes, and the onset of cannabis use.', 'Data are from the Mater-University Study of Pregnancy (MUSP), a 21-year prospective study in Brisbane, Australia. The present study is based on the 3008 mothers and their children for whom there were complete follow-up data at 21 years. Outcomes were self-reported cannabis use at age 21 and early onset (before age 15) cannabis use as judged from a retrospective report obtained at 21 years. Analyses were conducted using multivariate binomial and multinomial logistic regression.', 'Change in maternal marital status when the child was aged between 5 and 14 years was significantly associated with increased risk of cannabis use [odds ratio (OR) = 1.7; 95% confidence interval (95% CI) 1.4-2.0 for one or two marital changes and OR = 2.3; 95% CI 1.5-3.4 for three or more marital changes], after adjustment for a range of potential confounders.'] Are changes in maternal marital status associated with young adults ' cannabis use : evidence from a 21-year follow-up of a birth cohort?