Background Narrow-band imaging (NBI) makes it possible to emphasize the imaging of certain features, such as mucosal structures and mucosal microvessels in GI-tract diseases. Recently, video peroral cholangioscopy (POCS) was developed as a diagnostic modality for better visualization of bile-duct lesions; however, there is no report on POCS by using NBI. Objective To evaluate the clinical usefulness of POCS by using NBI for the diagnosis of biliary-tract diseases. Design Prospective case study. Setting This procedure was performed at Tokyo Medical University Hospital. Patients Twelve consecutive patients with biliary-tract diseases, including 7 bile-duct cancers and 5 benign biliary diseases, which revealed 6 bile-duct strictures and 6 filling defects by ERCP. Intervention All patients underwent POCS by using NBI. Main Outcome Measurement Efficacy and safety of this technique. Results Twenty-one lesions were evaluated by using POCS with conventional white light imaging and NBI. Although visualization of only 2 lesions (9.5%) was “excellent” by conventional observation, visualization of 12 lesions (57.4%) was “excellent” by NBI observation. Identification of the surface structure and vessels of the lesions by NBI observation was significantly better than with conventional observation ( P < .01 and P < .05, respectively). Limitations Maneuverability and fragility of POCS. The current POCS is not equipped with magnification. Conclusions POCS by using NBI may be helpful for the observation of both fine mucosal structures and tumor vessels.