Hypertrophic cardiomyopathy is characterized by both increased ventricular wall thickness and impairment to left ventricular filling. However, the interrelation between these two pathologic features of the disease is not well understood. To investigate the relationship between diastolic abnormalities and left ventricular hypertrophy, 52 patients with hypertrophic cardiomyopathy and 22 normal subjects were studied with digitized M-mode echocardiography and two-dimensional echocardiography. Echocardiographic indexes of diastolic function were compared in patients with different extent of left ventricular hypertrophy. Left ventricular relaxation and time to peak rate of increase in left ventricular dimension were prolonged (80 plus/minus 31 and 100 plus/minus 37 msec) in patients with hypertrophic cardiomyopathy and severe left ventricular hypertrophy, compared to patients with mild left ventricular hypertrophy (59 plus/minus 25 and 74 plus/minus 34 msec; p less than 0.01). However, left ventricular relaxation and time to peak rate of increase in left ventriuclar dimensions were also abnormal in patients with mild left ventricular hypertrophy, compared to controls (24 plus/minus 12 and 49 plus/minus 12 msec; p less than 0.01). Furthermore, in 32 patients with hypertrophic cardiomyopathy who had normal posterior wall thickness, peak rate of posterior wall diastoclic excursion was significantly reduced (11 plus/minus 3 cm/sec) compared to controls (16 plus/minus 2 cm/sec; p less than 0.01). In conclusion, mildly hypertrophied left ventricle and even non-hypertrophied left ventricular segments may demonstrate diastolic abnormalities in patients with hypertrophic cardiomyopathy. This finding suggests that the primary cardiomyopathic process in hypertrophic cardiomyopathy is not limited to areas of wall thickening.