To determine whether thallium 201 myocardial imaging could be used to detect hypertrophic cardiomyopathy, 10 patients with idiopathic hypertrophic subaortic stenosis were studied. Asymmetric septal hypertrophy, an anatomic marker for this disease, was evident on thallium scans in the patients with idiopathic hypertrophic subaortic stenosis, with a ratio of septum to left ventricular free wall of 1.7 cm compared to 1.0 cm in eight normal volunteers, and 1.0 cm in eight patients with concentric left ventricular hypertrophy. Patients with hypertrophic cardiomyopathy also demonstrated apical ventricular wall thickening, and a thicker posterior ventricular free wall in the obstructive in contrast to the nonobstructive form of this disease. Thallium scans of three patients with pulmonary hypertension and secondary asymmetric septal hypertrophy were distinguished from idiopathic hypertrophic subaortic stenosis by marked right ventricular wall thickening. Thus, thallium 201 myocardial perfusion imaging provides a noninvasive method for detection and evaluation of idiopathic hypertrophic subaortic stenosis.