The frequency, severity, and cause of aortic regurgitation were assessed by colour Doppler and cross sectional echocardiography in 87 patients (mean SD) age 57 (12) years) with hypertrophic cardiomyopathy, and 48 age matched controls (57 (8) years). Aortic regurgitant murmurs were recorded in only three of 87 patients and in none of the controls. Colour Doppler echocardiography showed an aortic regurgitant signal in 20 (23%) of the patients and three (6%) of the 48 controls. The colour Doppler signals typical of aortic regurgitation were limited to the left ventricular outflow tract. There were no significant differences between patients with hypertrophic cardiomyopathy with and without aortic regurgitation in terms of age (59 years v 56 years), blood pressure (140/84 mm Hg v 136/80 mm Hg), aortic diameter (34 mm v 33 mm), or frequency of calcification of the aortic valve (15% v 10%) and of systolic anterior motion of the mitral valve with mitral-septal contact (25% v 16%). On cross sectional echocardiograms, the degree of septal protrusion into the left ventricular outflow tract during systole was significantly more prominent (15 v 10 mm), and the portion of the basal septum that protruded most deeply into the left ventricular outflow tract was significantly closer to the aortic annulus in patients with aortic regurgitation than in those without it (11 v 14 mm). Mild aortic regurgitation was found in almost a quarter of patients with hypertrophic cardiomyopathy. The regurgitation was related to the morphological abnormality of the left ventricular outflow tract.