Of 89 patients with idiopathic hypertrophic subaortic stenosis who had M-mode echocardiagrams recorded, seven patients with coexisting moderate to large pericardial effusions were identified. The clinical profile, M-mode echocardiograms, and cardiac catheterization and angiocardiographic studies in two of the seven patients were analyzed. That the two entities were not associated was suggested by the identification of an etiology for the pericardial effusion in four of the patients. Although the "swinging heart" phenomenon was exhibited in the echocardiograms of each patient, the presence of a significant pericardial effusion did not preclude the ability to establish a diagnosis of idiopathic hypertrophic subaortic stenosis by M-mode echocardiography.