Accurate preoperative diagnosis of renal angiomyolipomas is essential if conservative resection or angiographic embolization is to be used. Computed tomographic (CT) scanning failed to diagnose angiomyolipoma preoperatively in 2 patients, and this led to a retrospective review of 6 patients to define the limits of CT scanning in preoperative diagnosis. The CT findings were correlated with the histology of the tumors. Two tumors with positive attenuation coefficients suggestive of renal adenocarcinoma had significant amounts of immature "fetal" fat, and one of these had a virtual absence of mature fat. The relative inability of CT scanning to identify immature "fetal" fat, especially in the presence of abundant vessel and muscle elements, is a limitation to its use in the preoperative diagnosis of angiomyolipoma. A negative attenuation coefficient is highly characteristic of renal angiomyolipoma with mature fat elements. A positive attenuation coefficient, although suggestive of renal cell carcinoma, may be found in angiomyolipomas with paucity of mature fat or high proportions of immature fat.