In order to study left ventricular filling in hypertrophic cardiomyopathy, left ventricular cineangiograms of 20 patients were digitised frame by frame and compared with those of 10 normal subjects. Peak left ventricular filling rate was 770+/-260 ml s-1 (mean+/-1 standard deviation), not significantly different from normal. Mitral valve opening was delayed, occurring 140+/-40 ms after time of minimum cavity area, compared with 93+/-14 msec in normals (P less than 0-01), indicating abnormal relaxation. Peak left ventricular filling rate was correlated inversely with this delay (r = 0-69, P less than 0-01), which was greater in patients with angina (155+/-27 ms) than in those without (85+/-23 ms) (P less than 0-01). The rapid filling period was also abnormally prolonged in 8 patients. End-diastolic transverse dimension was normal (5-3+/-0-7 cm) but end-systolic dimension was reduced (2-4+/-0-4 cm) (P less than 0-01) because of abnormal cavity shape. Peak rate of change of dimension during filling (18-7+/-5-3 cm s-1) was greater than normal (11-3+/-3-9 cm s-1) (P less than 0-01), and correlated with peak filling rate (r = 0-82, P less than 0-001). Thus, peak filling rate is normal in hypertrophic cardiomyopathy, but the filling pattern may be abnormal, apparently because of impaired relaxation and abnormal cavity shape rather than mechanical obstruction to inflow.