Doppler echocardiography was used to assess diastolic function in 40 patients with hypertrophic cardiomyopathy and to relate it to the patients' symptoms, anaerobic threshold and maximal oxygen consumption during cardiopulmonary exercise testing. The patients had a smaller early (E wave) (p less than 0.01), higher late (A wave) (p less than 0.05) mitral diastolic flow velocity, larger A/E ratio (p less than 0.01), longer isovolumetric relaxation time and E wave duration (p less than 0.001) and slower deceleration rate of the E wave (p less than 0.001) than 40 age- and gender-matched normal subjects. In the patients with hypertrophic cardiomyopathy, maximal oxygen consumption and anaerobic threshold were, respectively, 26.3 +/- 9.2 and 21.1 +/- 6.1 ml/kg per min compared with 47 (range 39 to 68) (p less than 0.01) and 41 (range 27 to 58) ml/kg per min (p less than 0.01) in normal subjects. There was no relation between Doppler indexes and symptoms but symptomatic patients had lower maximal oxygen consumption and anaerobic threshold compared with asymptomatic patients (21.4 +/- 7 vs. 30.7 +/- 10, p less than 0.001 and 18.6 +/- 4.7 vs. 23.1 +/- 5.7, respectively, p less than 0.001). In conclusion, Doppler echocardiography can identify abnormalities of left ventricular filling in patients with hypertrophic cardiomyopathy. However, these indexes measured at rest do not correspond to the patient's professed symptomatic status or exercise capacity measured objectively. Conversely, cardiopulmonary exercise testing reveals a depressed maximal oxygen consumption and anaerobic threshold even in the least symptomatic patients.