Computed tomographic (CT) scans of 34 cases of abdominal fatty masses were reviewed retrospectively in order to establish criteria for distinguishing benign conditions from malignant tumors. By evaluating location, attenuation, internal consistency, and margination, it is possible not only to make this distinction but frequently to suggest a specific diagnosis. Abdominal fatty masses that are sharply marginated, homogeneous, and that show CT numbers less than or equal to the patient's normal fat can be considered benign. Malignancy should be suspected when an extrarenal abdominal fatty mass displays one or more of the following characteristics: inhomogeneity, infiltration or poor margination, CT numbers greater than the patient's normal fat, or contrast enhancement. Criteria for distinguishing between angiomyolipoma and liposarcoma are also discussed.