Patients with chronic renal failure, especially those receiving maintenance haemodialysis, have a number of dermatological alterations. Some of them are similar to those seen in porphyria cutanea tarda (PCT). Whereas early studies showed normal plasma porphyrin levels, a striking elevation of plasma porphyrins, and particularly of uroporphyrin, has recently been found. We measured the concentrations of porphyrins in the plasma and erythrocytes of 55 patients with chronic renal failure and receiving maintenance haemodialysis and also in 7 patients with PCT and 100 healthy volunteers. The mean concentration of porphyrins in plasma was 2.7-fold, and the maximum concentration, 6-fold the highest value measured in plasma of controls. The mean plasma concentration of uroporphyrin was 6-fold higher up to a maximum value of 37-fold the upper limit of the controls. The plasma porphyrin values of 3 of 7 patients with PCT were on the same high level as those measured in patients undergoing haemodialysis. The mean porphyrin concentration in the erythrocytes of haemodialysis patients were 1.5-fold the control values. Because of the known pathophysiological effect of uroporphyrin, especially its stimulation of the collagen synthesis and the activation of the complement system, we suppose that the porphyria cutanea tarda like skin lesions in patients with chronic renal failure are due to the highly increased uroporphyrin concentration.