Hemodialysis is associated with acute reduction in intravascular volume. To assess the impact of volume reduction on left and right ventricular diastolic filling indexes obtained by Doppler echocardiography, 24 patients on chronic hemodialysis were consecutively studied before, during, and immediately after one hemodialysis session. Twenty four normal sex and age-matched volunteers served as a control group. Study patients had abnormal diastolic indexes when compared to controls. At 2 hours of dialysis (mid dialysis) there was a significant decrease in peak early mitral flow velocity (E), no change in peak atrial filling velocity (A), and a reduction in the E/A ratio. The deceleration time of the mitral E wave also was prolonged compared to baseline. Similar findings were observed with respect to right ventricular filling indices. These changes occurred during the first 2 hours of dialysis and remained unaltered at end dialysis. When patients were subdivided according to weight loss, only the group that lost 1 or more kilograms had significant changes in the Doppler parameters of the left and right ventricle, as well as reduction of the left ventricular dimensions. These findings suggest that preload reduction is the main mechanism that accounts for acute changes in Doppler diastolic indices observed during hemodialysis.