Purpose: This study was undertaken to determine the efficacy of polyurethane covered metallic stent in the treatment of patients with distal malignant biliary obstruction. Patients and Methods: From August 1998 to October 1999, we enrolled consecutive 50 patients with irresectable biliary malignancy of common bile duct. They were randomly assigned to polyurethane covered Diamond stent group (25) or original Diamond stent group (25). The prosthesis were implanted endoscopically. Stent patency represented from stent insertion to its occlusion or patient death. Cumulative stent patency and patient survival were estimated according to Kaplan-Meier analysis, supplemented by the log-rank test. Results: There was no significant difference between groups in patient survival. Effective biliary decompression was achieved in all patients initially. The stent patency rates at 3, 6, and 12 months were 100%, 83%, and 83% in the covered group, and 77%, 69%, and 52% in the uncovered group (p=0.055) respectively. Stent occlusion occurred in 1 patient (4%) in the covered group, and in 7 patients (28%) in the uncovered group, respectively. The cause of covered stent occulusion was tumor overgrowth over the tips of the metallic stent. On the other hand, the causes of occlusion in the uncovered group consisted of tumor ingrowth through the stent meshes in 5, tumor overgrowth in 1 and unknown cause in 1. Acute cholecystitis in 1 and mild pancreatitis in 3 occurred in the covered group. Percutaneously drainage was needed for management of cholecystitis. Pancreatitis conservatively improved. In the uncovered group, those complications did not occur. However, biliary bleeding in 1 occurred in uncovered group. Abdominal arteriography showed pseudoaneurysm in common bile duct and transcatheter arterial embolization with gelfoam was performed. Conclusions: Covered Diamond stent showed higher patency rates than the uncovered Diamond stent. Polyurethane cover may be effective to prevent against tumor ingrowth. However, this study is preliminary in nature and further investigations will be needed.