There were 62 patients referred to our institution for evaluation of upper urinary tract filling defects. These patients had undergone any of several diagnostic evaluations, including computerized tomography, B-mode ultrasound scanning and urinary cytology studies with or without upper tract brush biopsy. In all cases a conclusive diagnosis could not be formulated. With the use of flexible ureteropyeloscopy complete evaluations of the upper tract lesions were performed. The upper tract endoscopic procedure provided the diagnosis in all patients, as well as allowed for therapeutic intervention in 19. There was no significant morbidity associated with any procedure. We advocate the continued development of flexible ureteropyeloscopic instrumentation and procedures. We suggest the inclusion of ureteropyeloscopy with tissue sampling of neoplasms in the evaluation of all upper urinary tract lesions, except when diagnostic ultrasound correlates the characteristics and position of the lesion as calculous disease.