Forty children with chronic renal failure (CRF) on conservative treatment, on hemodialysis, or after renal transplantation and 22 children respresenting a non-uremic control group were subjected to repeated cardiologic examinations by ECG, PCG, chest X-rays and cycle ergometer exercise tests to monitor signs of uremic heart disease and to evaluate physical working capacity (W170). In the CRF group a progressive impairment of W170 was found, starting at an early stage of the disease. Exercise tolerance was inversely related to the degree of CRF. A correlation was also found between W170 and renal anemia. After starting dialysis, W170 failed to increase significantly. Immediately after dialysis an acute drop in W170 occurred. Renal anemia was found to be the main pathogenetic factor of uremic heart disease in children. In some cases hypercirculation following arteriovenous fistulae became equally important as a cause of reduced myocardial performance. Physical rehabilitation, as measured by exercise tolerance tests, was better in transplanted than in dialysed children.