The porphyrin status of 38 renal transplant patients was investigated using a sensitive thin-layer chromatographic assay. Abnormalities of porphyrin synthesis observed in patients on hemodialysis were generally reversed on receipt of a transplant with sustained good renal function. Urinary coproporphyrin levels and the creatinine clearance appeared directly related in both individual cases and generally, substantiating the theory that coproporphyrin is of renal origin. The pre- and postoperative porphyrin status of a unique patient with variegate porphyria who received a successful renal transplant implies that the excess urinary porphyrins and precursors excreted in the acute porphyrias are also of mainly renal, as opposed to hepatic origin. Furthermore, the posttransplant overproduction of porphyrins by a kidney from a normal donor suggests the existence of an endogenous porphyrogenic agent in the circulation of patients with acute porphyria.