Previous reports have demonstrated that patients with hypertrophic cardiomyopathy (HCM) have prolonged isovolumic relaxation period (IRP) reflecting reduced rate of fall of left ventricular pressure. Eighty four patients with proven hypertrophic cardiomyopathy and 31 normal subjects were studied by simultaneous recordings of echocardiogram, apexcardiogram, phonocardiogram and ECG. In normal subjects the IRP value was 61 +/- 11 ms (mean +/- s.d.). In the 84 patients there was enormous variability of the IRP value from 0 to 160 ms reflecting abnormal and incoordinate (but not necessarily impaired) relaxation and it was possible to identify three subgroups among the patients: 60 patients in sinus rhythm who had prolonged IRP and significantly above the normal values, 9 patients in atrial fibrillation in whom the IRP was within the normal range and 15 patients with IRP values between 0-45 ms, with the mean (26 ms) below the normal range (mean +/- 2 s.d.). This group of patients with short IRP also had signs of outflow tract systolic pressure gradient, with partial mid-systolic closure of the aortic valve, systolic anterior motion of the anterior mitral valve leaflet and paradoxical splitting of the second heart sound. It is suggested that the short IRP is due to extremely delayed aortic valve closure and careful scrutiny of this subset with haemodynamic evaluation has shown that this non-invasive interval (A2-Mo) may not always be a valid measure of left ventricular relaxation.