To examine the relations among hypertrophy, fibrosis and diastolic performance in early experimental hypertension, 18 control dogs and 12 dogs with experimental left ventricular hypertrophy were studied. Diastolic function was impaired in dogs with left ventricular hypertrophy, with decreased Doppler early to atrial inflow velocity ratio (E/A) (1.35 versus 1.72), increased atrial filling fraction (35% versus 29%), decreased sonomicrometric peak rates of wall thinning (-2.01 versus -3.37 liters/s) and filling (4.33 versus 6.64 liters/s) and prolonged time constant of isovolumetric relaxation (tau; 34.3 versus 28.1 ms). Neither chamber stiffness (k; P = AekV) nor passive elastic stiffness (E; E = k sigma, where sigma = stress) was increased. At postmortem examination, the hypertensive left ventricle weighed significantly more than normal (116 versus 80 g; p less than 0.01) and had greater muscle fiber diameter at endocardial and epicardial sampling sites in the apical free wall, basal free wall and septum (mean diameter 50 +/- 8 microns in hypertensive dogs, 37 +/- 8 microns in normal dogs; p less than 0.01). In contrast, neither percent fibrosis (1.2 +/- 0.8 versus 0.9 +/- 0.6 in normal dogs) nor fibrotic volume (1.21 +/- 0.63 versus 0.72 +/- 0.42%/g in normal dogs) was significantly increased. Peak volumetric filling rate was inversely related to fiber diameter (r = -0.74, p less than 0.001), although no variable of left ventricular function was significantly related to percent or volume fibrosis (all r less than 0.60, all p greater than 0.05). Thus, diastolic dysfunction may exist in the setting of hypertrophy without significant fibrosis. Increased myocyte size was associated with early diastolic filling abnormalities characteristic of the hypertensive left ventricle. Fibrosis appears to be a less important determinant of diastolic performance.