Urinary coproporphyrin isomer distribution was studied in 13 patients suffering from the Dubin-Johnson syndrome and several control groups. In patients with the Dubin-Johnson syndrome the isomer distribution was quite the reverse to that of normal persons. The urinary isomer distribution was in the normal range in patients with Gilbert's syndrome, hemolytic jaundice, and fatty liver. In many patients with different acute and chronic liver diseases, a clear deviation from the normal urinary isomer distribution was seen. There was, however, no overlapping with the Dubin-Johnson syndrome. Even the fecal coproporphyrin isomer distribution deviated significantly from the normal in favor of Isomer I.