This study examines the association of symptoms of Lifetime antisocial personality disorder (ASP with marijuana abuse/dependence in Vietnam-era veteran male monozygotic twin pairs. In 1992, 1,874 monozygotic twin pairs responded to a structured psychiatric interview that obtained data on lifetime history of drug use and ASP. Among randomly selected individuals from each twin pair, 8 of 10 ASP symptoms were significantly more prevalent in persons with a lifetime history of marijuana abuse/dependence compared with those who had never abused any drug (p < .001). Among 99 marijuana discordant twin pairs, however, only two ASP symptoms, 'failure to conform to social norms' (odds ratio, 2.8; 95% confidence interval, 1.5 to 5.5) and 'reckless regard of own or other's personal safety' (odds ratio, 2.4; 95% confidence interval, 1.0 to 5.4) were Significantly increased in marijuana abusing/dependent twins compared with their non-abusing/nondependent twin brother. After adjustment for conduct disorder, alcohol abuse/dependence, and exposure to combat in Vietnam, only 'failure to conform to social norms of lawful behavior' (odds ratio, 2.42; 95% confidence interval, 1.12 to 5.21) remained significantly increased in twins with marijuana abuse/dependence.