Objective: In gastrointestinal endoscopy, the bleeding volume does not increase, even if a biopsy is performed under continuous warfarin administration. However, the safety when anticoagulant therapy is continued during flexible bronchoscopy has not been reported. We measured the actual blood loss in patients under continuous warfarin administration. Methods: This study was designed as a single-center, double-arm, prospective clinical trial. The bleeding volume during the procedure was calculated from aspirated secretions with a portable hemoglobin-measuring device. Results: Twenty-three subjects in the control group and 5 in the warfarin group were enrolled over a 60-month period. Overall, the blood loss in the warfarin group was comparable to that in the control group ( median 0.2 vs. 0.8 ml ) . No subject experienced severe blood loss. All patients in the warfarin group used a guide sheath. Conclusions: Although limited to peripheral lesions with a guide sheath, we found no severe bleeding in the warfarin group.