35 patients with chronic renal failure were examined during the first 2 h of hemodialysis by invasive and non-invasive methods. The cardiac output was determined by the dye dilution technique in 11 of these patients and by Swan-Ganz catheterization in 5. In the patients without severe myocardial damage, the cardiac output during hemodialysis showed a significant increase (p < 0.05), accompanied by a significant decrease of both mean blood pressure (p < 0.05) and total peripheral resistance (p < 0.05). Mean pulmonary artery pressure and pulmonary artery wedge pressure were reduced in all cases during hemodialysis. The increase in cardiac output during hemodialysis in patients with chronic renal failure may be attributed to the decrease in afterload.