Raised plasma uroporphyrin levels were found in all the 14 patients with end-stage renal disease studied, 7 of whom were on continuous ambulatory peritoneal dialysis (CAPD) and 7 on hemodialysis (HD). The elevation observed in the HD group was higher than that noted in the CAPD group; 18-fold in the former and 13-fold in the latter. The difference in uroporphyrin plasma levels between the two dialysis populations might be explained, at least partially, by the reduced activity of uroporphyrinogen decarboxylase (UROD), the enzyme which converts uroporphyrinogen to coproporphyrinogen. A decrease of 48% was noted in the HD group, whereas no change was observed in the CAPD group. A significant negative correlation (r = -0.37, p < 0.01) was found between the concentration of uroporphyrin in plasma and the activity of UROD. In view of the data shown, it is suggested that erythrocyte UROD activity should be interpreted with caution in HD patients suspected of having porphyria cutanea tarda.