Data from the NHLBI Twin Study were used to investigate the genetic component in a number of Type A measures given to these twins during a second cardiovascular examination. Specifically, the objective of the current study was to determine the extent to which various Type A measures are influenced by genetic effects and by measurable environmental and cultural factors. Analyses of these data for twins yielded a number of results. First, the Type A behavior pattern as assessed by the structured interview was only weakly associated with self-report inventories developed as alternatives to the structured interview. Second, among the self-report measures of Type A, only the Thurstone Temperament Schedule Active scale showed a clear significant genetic component. Most important, a number of demographic and social characteristics known to be associated with the various Type A scales had a differential effect on twin similarities. Specifically, for the job involvement subscale of the Jenkins Activity Survey, twins of both zygosities became equally similar after adjustments for covariates, while no effect on twin similarity was noted for the Thurstone Temperament Schedule Active scale. It is concluded that a complex constellation of genetic predispositions and acquired behaviors underlies the Type A behavior construct.