Background: The necessity for drainage of both liver lobes in tumors arising at the biliary bifurcation is controversial. The aim of this study was to compare the outcome of unilateral versus bilateral drainage in patients with biliary obstruction at the hilum. Methods: One hundred fifty-seven consecutive patients with primary cholangiocarcinoma, gallbladder cancer, or periportal lymph node metastases were randomly allocated to unilateral (group A) or bilateral (group B) hepatic duct drainage. Results: In intention-to treat analysis, group A had a significantly higher rate of successful endoscopic stent insertion than group B (88.6% vs. 76.9%, p = 0.041). Group B had a significantly higher rate of complications than group A (26.9% vs. 18.9%, p = 0.026) because of the higher rate of early cholangitis (16.6% vs. 8.8%, p = 0.013). In per-protocol analysis the rate of successful drainage, complications, and mortality did not differ between the two groups. Median survival did not differ between the two groups but was significantly different for patients with cholangiocarcinoma and those with gallbladder cancer versus patients with metastatic tumors (p = 0.0247). Conclusion: The insertion of more than one stent would not appear justified as a routine procedure in patients with biliary bifurcation tumors. (Gastrointest Endosc 2001;53:547-53.)