We studied the physical fitness of 71 patients with renal disease before dialysis, after starting dialysis, and after successful renal transplantation. Maximal muscle strength was determined in 58 patients and maximal cardiovascular capacity was assessed in 36 patients. Before dialysis there was a 31 percent reduction in muscle strength in men and 47 percent in women. The loss of muscle strength was most pronounced in the leg extensors. Cardiovascular capacity and muscle strength was positively correlated in the dialysis patients (p less than or equal to 0.05). The cardiovascular capacity was decreased 29% in predialysis patients, 45% in dialysis patients and 37% in transplanted patients compared to untrained healthy controls. The predialysis patients had a 19% greater cardiovascular capacity than the dialysis patients and a 9% greater cardiovascular capacity than the transplanted patients. While hemoglobin concentrations showed a positive correlation (p less than or equal to 0.05) with maximal cardiovascular capacity and creatinine concentration showed a negative correlation (p less than or equal to 0.05) with maximal cardiovascular capacity in the predialysis patients, hemoglobin concentrations and creatinine concentrations did not correlate with cardiovascular capacity in the dialysed and transplanted patients. In dialysis patients as well as in patients after kidney transplantation hemoglobin and creatinine concentrations, muscle strength, height, weight, and age are factors influencing the physical work capacity, but none of these variables alone seems to be a strong determination factor. Besides the complications of kidney disease which may itself reduce physical fitness, the lack of physical training is an important factor in determining the residual muscle strength and cardiovascular capacity for all patients with renal insufficiency.