Thirty-one men, originally seen as children for problems with overactivity, distractibility, and impulsiveness, were reevaluated at ages 21-23 years. Subjects were interviewed and tested and provided a blood sample for analyses of dopamine-beta-hydroxylase (DBH), monoamine oxidase (MAO), and catechol-O-methyl-transferase (COMT). A series of Pearson correlations between the biological variables and psychological measures revealed that MAO levels were negatively associated with measures of drug involvement, cigarette use, fire-setting, and sensation seeking; DBH levels were positively associated with measures of sensation-seeking; and COMT levels were negatively associated with a measure of hostility and positively correlated with a measure of impulsiveness.