This article examines whether young individuals in the general population with comorbid alcohol use and mental health disorders experience worse internalising and externalising behaviour problems than those with single disorders. A large cohort of women at the Mater Misericordiae Hospital in Brisbane, Australia, was enrolled during pregnancy in a longitudinal study. Mother/offspring dyads were followed over twenty-one years. At age 21, offspring behaviour problems were examined using the Young Adult Self Report, alcohol and mental health disorders with the Composite International Diagnostic Interview. Associations between comorbidity and behaviour problems were assessed using multinomial logistic regression, accounting for life-course factors. Twelve percent of young adults had alcohol/mental health DSM-IV disorders with significant temporal overlap. A further 16% had alcohol disorders only and 23% mental health disorders only. The comorbid group scored significantly higher on total and externalizing behaviour problems but not internalizing behaviour problems. Stronger associations of aggression/delinquency with comorbidity were not fully accounted for by factors known to influence separate development of mental health and alcohol disorders. Young adults with comorbid alcohol/mental health disorders experience more, and more severe, behavioural problems than those with single disorder types, indicating an increased burden from comorbidity, with implications for treatment and public order.