A review is presented of qualitative and quantitative morphologic findings in the hearts of patients with hypertrophic cardiomyopathy, with emphasis on the significance of the amount of myocardial fiber disarray present in such patients and in patients with other types of heart disease unrelated to hypertrophic cardiomyopathy. A new theory of morphogenesis is proposed to account for the asymmetric septal hypertrophy and the fiber disarray that occur in hypertrophic cardiomyopathy.