In hypertrophic cardiomyopthy, the relation between left ventricular diastolic impairment and magnitude of left ventricular hypertrophy has not been clearly defined. In the present study, Doppler echocardiographic indexes of left ventricular diastolic filling were compared in 78 patients with hypertrophic cardiomyopathy and in 72 normal control subjects of similar age, and the relation between abnormalities of diastolic filling and magnitude of left ventricular hypertrophy was assessed. In patients with hypertrophic cardiomyopathy, isovolumic relaxation was prolonged (94 ± 25 ms); peak early diastolic flow velocity (53 ± 18 cm/s), deceleration of flow velocity in early diastole (341 ± 142 cm/s2) and the ratio between early and late peaks of flow velocity (1.6 ± 0.9) were reduced; and peak late diastolic flow velocity was increased (38 ± 15 cm/s) compared with values in control subjects (76 ± 12 ms, 65 ± 12 cm/s, 512 ± 131 cm/s2, 2.3 ± 0.8 and 30 ± 7 cm/s, respectively; p < 0.001). Individual patient analysis showed that diastolic filling was abnormal in 52 (67%) of the 78 patients with hypertrophic cardiomyopathy. However, within the patient group, none of the Doppler diastolic indexes showed a significant correlation with maximal left ventricular wall or the wall thickness index (correlation coefficients ranged from -0.15 to 0.10). Furthermore, mean values for each of the Doppler diastolic indexes were similar and not significantly different in the 24 patients with only mild, localized hypertrophy (wall thickness of 15 to 17 mm in only one ventricular segment) compred with the 8 study patients with particularly marked and diffuse hypertrophy (maximal wall thickness ≥30 mm or wall thickness ≥25 mm in two or more segments); individual patient analysis showed that abnormal filling patterns were similarly common in patients with mild hypertrophy (17 of 24, or 71%) and in those with marked hypertrophy (5 of 8, or 63%; p > 0.05). These results indicate that diastolic filling abnormalities in hypertrophic cardiomyopathy are largely independent of the magnitude of left ventricular hypertrophy and are common in patients with only mild, localized hypertrophy. These findings also suggest that the primary myopathic process responsible for the impaired diastolic filling in this disease is not confined to the thickened portions of the ventricular wall. © 1990.