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Rosenman and colleagues reported no heritability of global Type A behavior assessed by the Structured Interview (SI) method, although some of the self-report scales correlated with global Type A behavior did have heritable components. Recent factor analyses of coded SI responses revealed four independent dimensions: clinical ratings, primarily of speech stylistics; and self-reports of pressured drive, anger, and competitiveness. It may be that some of these dimensions have a heritable base, whereas others do not. We report here reanalyses of the available SI responses from the Rosenman sample. In this subsample, tape recorded interviews with 80 monozygotic and 80 dizygotic twin pairs were scored for the extent of self-reported Type A behaviors, the major speech stylistics considered to be indicative of Pattern A, and the observable Type A behaviors (e.g., signs of hostility). Then scores for the major dimensions measured by the SI were calculated and scores for which there were sufficient data were subjected to twin analyses by the method of Christian et al. These analyses showed that individual differences in the clinical ratings factor and certain ratings loading on it--specifically, loudness of speech, competition for control of the interview, and potential for hostility--might have a heritable component. These results are discussed in the context of the importance of hostility as a predictor of subsequent coronary heart disease events as well as of total mortality, other data on the heritability of emotionality, and a temperament approach to understanding the origins of the Type A behavior pattern.