We evaluated 12 patients with unilateral unexplained gross hematuria by flexible ureteropyeloscopy and percutaneous pyeloscopy. All patients had localized bleeding except for 1 with diffuse bleeding caused by the nutcracker phenomenon, and 2 in whom no hematuria appeared upon examination and no gross lesions were observed. Among the 9 patients with localized bleeding transitional cell carcinoma was found in 1, hemangioma in 4 and minute venous rupture in 4. These 9 patients were treated endoscopically and no recurrences were observed during a follow-up of 6 to 21 months (average 10.3 months). Our results underscore the importance and efficacy of flexible ureteropyeloscopy in the evaluation and management of chronic unilateral hematuria.