Median survival time of nonresectable hilar bile duct cancer is only 4 to 6 months owing to tumor spread in the biliary tree, refractory cholestasis, and sepsis or liver failure. We explored whether local photodynamic therapy of nonresectable bile duct cancer could improve survival. A sample size of 23 patients is required to detect an increase in 6-month survival rate from less than 50% to greater than 70% in a single-arm phase-II trial with a statistical power of 80% (Fleming's single step procedure; ␣ ؍ 0.05). Twentythree consecutive patients (8 women, 15 men; 67 ؎ 14 years) with nonresectable bile duct cancer (Bismuth type III n ؍ 2, type IV n ؍ 21) were treated with photodynamic therapy and biliary endoprosthesis. Photofrin (QLT Pharmaceuticals, Vancouver, Canada) (2 mg/kg body weight intravenously) was photoactivated after 1 to 4 days with laser light (630 nm; 242 J/cm 2 ) via endoscopic retrograde access. The 6-month survival rate was 91% after diagnosis and 74% after start of photodynamic therapy (30-day mortality rate was 4%) at a median follow-up time of 10.3 months after diagnosis. Causes of death were tumor progression (n ؍ 9) and bacterial infections (n ؍ 4). The median rate of local tumor response was 74%, 54%, 29%, and 67% after the first, second, third, fourth, and fifth photodynamic therapy. Time to progression ranged from 3 to 8 months. All patients, except 1 with diffuse liver metastases, improved in cholestasis, performance, and quality of life. Photodynamic therapy can prevent tumor occlusion of hilar bile ducts. The apparent benefit in survival time should be confirmed in a controlled trial versus palliation by endoprosthesis only. (HEPATOLOGY 2000;31:291-298.)Hilar bile duct cancer is fraught with a very poor prognosis. 1,2 Tumors limited to the common bile duct, the bifurcation, or the bifurcation and the intrahepatic ducts of either the right or left lobe, types I, II, IIIa, and IIIb according to Bismuth, 3 may be curatively resectable with a 5-year survival rate of 9% to 21%, 4-8 and 26% to 30% in selected series. 9,10