Rationale:Occult small bowel bleeding is always difficult to locate by either radiological examination or endoscopy. When the site of bleeding cannot be found by nonsurgical measures, exploratory laparotomy becomes necessary.Patient concerns:A 63-year-old woman with a half-month history of occult gastrointestinal bleeding failed to many conservative therapies.Interventions:Intraoperative transillumination with water-filling of lumen was performed.Diagnoses:Small bowel bleeding was diagnosed intraoperatively.Outcomes:Segmental resection of the diseased small bowel with side-to-side anastomosis was performed while the histology showed no significant abnormality.Lessons:The technique of intraoperative transillumination with water-filling of lumen allows simple, accurate, and rapid localization of lesions in occult small bowel bleeding and facilitates precise and definitive surgery.