Acute renal infarction is rarely diagnosed before death despite fairly characteristic clinical and radiographic features. Definitive radiologic diagnosis of renal infarction often requires invasive procedures such as retrograde pyelography and angiography. The characteristic CT findings in three patients with proven renal infarction are reported. In these patients a CT rim sign was seen: a higher-attenuation subcapsular rim surrounded renal parenchyma of lower attenuation on contrast-enhanced scans. The appearance of renal infarction varies with both the extent and age of infarction. To assess these variables, segmental, major, and total renal infarcts were created in a series of six dogs; CT scans were obtained within 2 hr after infarction, and 1, 2, 4, and 8 weeks later. In all animals the ischemic and subsequently infarcted areas were seen on the initial examination as regions of low attenuation compared to the enhanced kidney. The low-attenuation areas markedly diminished by 2 weeks after infarction. CT should be useful in the prompt and noninvasive diagnosis of renal infarction.