Ninety-three pairs of monozygotic and 97 pairs of dizygotic middle-aged, American, male twins were studied to estimate the heritability of Type A behavior—a coronary heart disease-prone behavior pattern. Participants were given the interview assessment of Type A behavior. In addition, they completed four psychological test batteries: the Thurstone Temperament Schedule, the Jenkins Activity Survey, the California Psychological Inventory, and the Gough Adjective Check List. Type A behavior measured by interview was found to be nonheritable. Psychological test scales which significantly correlated with Type A behavior, however, generally had significant heritability estimates. Only the Adjective Check List scales, which significantly correlated with Type A, showed nonsignificant heritability estimates. Selected items from these Adjective Check List scales may provide a method for assessment of Type A behavior possibly free from genetic influence.