Left ventricular (LV) diastolic function may be affected early in patients with hypertrophic cardiomyopathy (HCM), regardless of the phenotypic expression of the disease. The aim of the present study was to detect whether LV diastolic performance, evaluated by conventional Doppler echocardiography, is impaired in first-degree relatives of patients with phenotypically expressed HCM, who had no clinical, electrocardiographic, or echocardiographic signs of the disease. Twenty-two young adults having the previously described characteristics comprised the study population and 22 sex- and age-matched healthy individuals served as controls. The 2 groups were compared according to several echocardiographic parameters and the following diastolic function indices: peak velocity of E wave, representing early filling; peak velocity of A wave, representing late filling; ratio of peak early to peak late velocity (E/A); deceleration time of E wave; and LV isovolumic relaxation time. Slower deceleration time of transmitral early filling in first-degree relatives of patients with HCM (192 +/- 31 vs 149 +/- 31 msec, p < 0.001) was the only variable that significantly differentiated the 2 groups. This study shows that in healthy persons with a family history of HCM, Doppler-derived mitral filling pattern shifted toward that observed in HCM and the slower deceleration time may serve as an early sign of disease development.