A functional VNTR polymorphism in the promoter of the monoamine oxidase A gene (MAOA-LPR) has previously been shown to be an important predictor of antisocial behavior in men. Testosterone analogues are known to interact with the MAOA promoter in vitro to influence gene transcription as well as in vivo to influence CSF levels of the MAO metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) in human males. We examined the possible joint effects of testosterone (measured in CSF) and MAOA-LPR genotype on antisocial personality disorder and scores on the Brown-Goodwin Aggression scale in 95 unrelated male criminal alcoholics and 45 controls. The results confirm that MAOA genotype and CSF testosterone interact to predict antisocial behaviors. The MAOA/testosterone interaction also predicted low levels of CSF MHPG, which tentatively suggests the possibility that the interaction may be mediated by a direct effect on gene transcription. If replicated these findings offer plausible explanations for previous inconsistencies in studies of the relationship between testosterone and male human aggression, as well as for how MAOA genotype may influence aggressive behavior in human males.