A 57-year old woman with a left renal aneurysm in a solitary kidney was successfully treated by in situ aneurysmectomy under careful renal preservation (intermittent perfusion combined with topical cooling). In our knowledge, 219 cases of renal aneurysms have been reported in the Japanese literatures by 1987. Among those cases, only 9 cases including our case were noted to have a solitary kidney. Surgical repair was recommended for renal aneurysm, especially, in a solitary kidney because of the high incidence of nephrectomy when ruptured.