During a 32-month period, 215 ureterorenoscopies were attempted in 171 ureterorenal units. The success rates for stone retrieval were 87% in the lower third of the ureter, 67% in the middle third, 75% in the upper third, and 55% in the pelvis. In 18% of cases a particular lesion could not be reached in diagnostic endoscopy but biopsies were obtained from seven out of eight tumours. The ureteric orifice was never dilated beyond 8 F and, yet, only four times could the instrument not be passed into the ureter. One major complication, an avulsion of 12 cm of the distal ureter, occurred. It is concluded that ureterorenoscopy is of significant value in the diagnosis and treatment of ureteric diseases.