A kindred with a high incidence of hypertrophic cardiomyopathy (HC) was studied to identify the presence of clinical hyperthyroidism. Two and possibly three members with HC had associated hyperthyroidism. In the propositus, clinical, ECG, and vectorcardiographic evidence for HC resolved after treatment of the hyperthyroidism. We hypothesize that the disproportionately high correlation of hyperthyroidism and HC and the resolution found in the propositus after treatment may indicate that antithyroid therapy should be considered in this form of cardiomyopathy.