The purpose of our study was to determine the value of unenhanced CT in the diagnosis of acute flank pain. We determined the accuracy of unenhanced Ct for stone detection as well as the detection of abnormalities unrelated to stone disease. During an 18-month interval, 292 patients with acute flank pain were imaged with unenhanced CT. Confirmation of the CT diagnosis was obtained for 210 patients: One hundred patients were proved to have ureteral stones based on other imaging studies (58 patients), lithotripsy (seven patients), ureteroscopic stone extraction (five patients), and stone recovery (30 patients). One hundred ten patients were proved not to have ureteral stones based on other imaging studies (24 patients), failure to recover a stone (56 patients), or a confirmed diagnosis unrelated to stone disease (30 patients). This latter group of 30 patients included diagnoses of adnexal masses (eight patients), appendicitis (five patients), diverticulitis (four patients), and common bile duct stones (three patients), as well as other diagnoses. Unenhanced CT findings were falsely negative for stone disease in three patients and falsely positive for stone disease in four patients. These data yield a sensitivity of 97%, a specificity of 96%, and an accuracy of 97% for diagnosing ureteral stone disease. Of 31 patients with a CT abnormality unrelated to stone disease, there was one false-negative diagnosis of acute appendicitis. Unenhanced CT is a valuable technique for examining patients with acute flank pain in whom a clinical diagnosis is uncertain. It can accurately determine the presence or absence of ureteral stones as well as extraurinary causes of acute flank pain. In most cases, other imaging studies are not required.