BACKGROUND:Many patients who require endoscopic treatments such as biopsy and polypectomy are given antiplatelet agents reluctantly. We have studied the effects of aspirin on colonic mucosal hemostasis.METHODS AND PATIENTS:We developed a new endoscopic device to make a standard incision (7‐mm length) on the colonic mucosa to study colon bleeding time. We measured the colon bleeding time of normal colonic mucosa in 47 cases. The colon bleeding time and skin bleeding time (Simplate method) were measured before and one hour after aspirin ingestion (990 mg) in ten healthy subjects.RESULTS:The bleeding time of normal colonic mucosa was 156±71 (mean±standard deviation) seconds. Significant prolongation was noted in both skin bleeding time (357±192vs.477±183 seconds;P<0.05) and colon bleeding time (155±47vs.244±169 seconds;P<0.05) after aspirin ingestion.CONCLUSIONS:Bleeding time was measured safely under direct colonoscopic visualization. Aspirin prolonged the colon bleeding time. Therefore, endoscopists should be aware of a risk of abnormal bleeding after endoscopic biopsy and polypectomy in patients with aspirin use. Two days were necessary for colon bleeding time to become normalized in patients with aspirin use.