A quarter of patients with erythropoietic protoporphyria develop mild to severe cholestatic liver disease. The determination of early indicators of hepatobiliary involvement are of pivotal importance to select patients for choleretic therapy. Porphyrin parameters were studied during ursodeoxycholic acid treatment in eight patients with protoporphyrin-associated liver disease and eight patients with liver failure before and after liver transplantation. The patients with intrahepatic cholestasis exhibited excessive protoporphyrinemia (27 μmol/l) compared with controls (normal<0.64 μmol/l). Fecal protoporphyrin excretion decreased in patients with deterioration of liver function, whereas urinary coproporphyrin increased up to 2290 nmol/24 h (normal <119 nmol/24 h). Coproporphyrin isomer I proportion increased to 71±10% (x̄±SD, n=8) in patients with terminal liver failure (normal <31%). During therapy with ursodeoxycholic acid biochemical improvement occurred but without clinical remission in most cases. Eight patients underwent liver transplantation between 1987 and 1997. One patient died of liver failure. Two transplant recipients are in a good condition since 8 and 9 years, respectively. All explanted livers revealed micronodular cirrhosis and high protoporphyrin levels of about 25,000-fold (x̄, n=3). Immediately after liver transplantation protoporphyrin in erythrocytes decreased to 46–96% of pre-operative values. Coproporphyrin remained moderately elevated due to postoperative cholestasis. A post-operative rise in fecal protoporphyrin elimination reflected sufficient biliary clearence of protoporphyrin by the transplant. In conclusion, moderate coproporphyrinuria with isomer I is the earliest sign of liver complications in erythropoietic protoporphyria. Progression of protoporphyrin induced toxic liver injury is indicated by excessive protoporphyrinemia and coproporphyrinuria with an isomer I proportion >71±10%, and reduction of fecal protoporphyrin excretion. Results suggest that therapy of intrahepatic cholestasis with ursodeoxycholic acid is only effective in the initial stages of liver disease in erythropoietic protoporphyria. In patients with severe cholestatic hepatic failure, liver transplantation is the treatment of choice.