Significant persistent or recurrent bleeding associated with renal arterial pseudoaneurysms was noted in three of 300 nephrostomy procedures. The angiographic diagnosis was made at 9, 6, and 21 days after nephrostomy. The lesions were successfully treated by embolization in two and by intimal dissection in one. The arterial tears were caused by a 16 gauge Teflon-sheathed needle inserted too deeply into the kidney. It is suggested that the lesions can be largely prevented by using only 22 gauge needles for renal puncture and dilating the skinny needle tract by a special guide wire exchanger. A renal angiogram should be obtained promptly if the urine continues to be grossly bloody 3-4 days after nephrostomy. Embolization of a bleeding segmental renal vessel is simple and safe, whereas a conservative approach may lead to severe complications.