The clinical course of 29 patients bearing 36 renal artery aneurysms was reviewed. The majority of the lesions (30 of 36 or 83%) were detected incidentally in the course of angiographic examination, mostly in the study of peripheral atherosclerotic arterial disease. Twenty-five lesions 2.0 cm or less in diameter, treated conservatively and observed during a period of time of from 1 to 17 years, remained clinically silent. In 10 patients (with 11 lesions) surgical treatment was employed. For eight patients the surgical treatment consisted of reconstructive excision and repair, with excellent results in all cases but one for a period of observation of from 1 to 17 years. In one patient aneurysmal dilatation of a vein bypass graft resulted in nephrectomy 2 years following operation. In the remaining two patients nephrectomy was required. Unless hypertension or pregnancy complicates the clinical picture, renal arterial aneurysms 1.5 cm or less in diameter can be observed safely by periodic angiography. Surgical repair of an aneurysm is recommended regardless of size if pregnancy cannot be avoided and hypertension is uncontrolled, and in aneurysms 1.5 cm or less in diameter which show an increase in size. A number of reconstructive vascular procedures have proved successful. Ex vivo repair may be an organ-saving technical aid.