Aims: To assess the effects of former heavy marijuana use on selected aspects of health. Design: A monozygotic co-twin control design was used to compare the health of former heavy marijuana using male monozygotic twins to that of their co-twins who never used marijuana significantly. Setting: In-person survey and questionnaires. Participants: Fifty-six marijuana use discordant monozygotic twin pair members of the Vietnam Era Twin (VET) Registry. Measurements: Current socio-demographic characteristics; current nicotine and alcohol use; life-time nicotine and alcohol abuse/dependence; past 5-year physical and mental health services utilization; and health-related quality of life. Findings: The mean number of days on which the marijuana user twin used marijuana in his life-time was 1085, while the non-marijuana user used marijuana a maximum of 5 days. Marijuana was last used a mean of 20 years previously. No significant differences were found between the former marijuana user twins and their siblings for current socio-demographic characteristics; current nicotine or alcohol use; life-time nicotine or alcohol abuse/dependence; past 5-year out-patient or emergency room visits, hospitalizations or medication use for medical problems; past 5-year mental health out-patient use or hospitalizations; or health-related quality of life. Conclusions: Previous heavy marijuana use a mean of 20 years earlier by a group of men who reported no other significant illicit drug use does not appear to be associated with adverse socio-demographic, physical or mental health adverse effects. The conclusions of the study are limited by possible participation and recall biases, relatively small sample size and the absence of a physical health examination.