Idiopathic hypertrophic subaortic stenosis (IHSS) may coexist with aortic valve stenosis (AVS). This combination of lesions is uncommon and may not be identified by the usual preoperative clinical evaluation and laboratory studies including cardiac catheterization. Surgical relief of AVS without concomitant septal myectomy to relieve unrecognized underlying IHSS may be followed by a low cardiac output, evidence of residual outflow tract obstruction, and death. Echocardiography (ECHO) will accurately identify IHSS and should be performed in all patients suspected of having AVS. This report describes the first three patients in whom the preoperative diagnosis of both AVS and IHSS was made and who subsequently had both lesions corrected at the same operation. All patients survived operation. The outflow tract gradient was abolished in two patients and significantly reduced in the third. All patients with this combination of lesions should be offered operation for the same indication as isolated aortic valve stenosis. The operation should include a septal myectomy as well as relief of the aortic valve stenosis.