Rhabdoid tumor of the kidney (RTK) is unique among childhood renal neoplasms in its frequent association with primary or metastatic CNS lesions. Previous reports suggest that RTK has characteristic CT features. It has been proposed that if CT could accurately predict the correct diagnosis of RTK preoperatively, then imaging protocols might be modified during the examination to include imaging of the brain. We wished to determine if RTK could be reliably distinguished from other renal neoplasms of early childhood. We retrospectively reviewed clinical, radiologic, and pathologic records of 21 patients with RTK. The study group included 13 males and eight females who were newborn to 36 months old (mean, 11 months). The study group was compared with 153 patients who were 3 years old or younger and who had solid renal masses. From the comparison group a subset of 54 patients 1 year old and younger was also selected for comparison with 13 (62%) of the 21 patients in the study group who were 1 year old or younger. Both comparison groups consisted of patients whose case material was consecutively added to the radiologic pathology archives at our institution. Diagnoses of the group of 153 patients were Wilms' tumor (n = 93), mesoblastic nephroma (n = 44), clear cell sarcoma of the kidney (n = 12), renal cell carcinoma (n = 3), and undifferentiated sarcoma (n = 1). A prominent eccentric crescent with the attenuation of fluid, representing sub-capsular renal hemorrhage or peripheral tumor necrosis adjacent to tumor lobules, was revealed on CT scans in 15 (71%) of the 21 patients with RTK and in seven (54%) of the 13 patients with RTK who were 1 year old or younger. Nineteen (12%) of 153 patients in the larger comparison group, representing patients with all tumor types, had CT features identical to those of the RTK group, including six (4%) patients with pathologic confirmation of sub-capsular renal hematomas. Six (11%) of the 54 comparison patients 1 year old and younger had CT features identical to those of the RTK group, and all proved to have mesoblastic nephroma. Associated CNS lesions were seen on CT or MR imaging in 11 (52%) of the 21 patients with RTK but none was seen in the comparison group of patients. On CT, a peripheral crescent with the attenuation of fluid is characteristic of RTK. However, 12% of renal neoplasms that occur more commonly than RTK in children had CT findings indistinguishable from those of RTK. Because the prevalence of RTK is relatively low, and because the CT findings are not pathognomonic, a renal mass seen on CT in a child is unlikely to represent RTK regardless of its CT features. We therefore conclude that the routine addition of CT of the brain for pediatric patients with renal masses that show a peripheral crescent of fluid attenuation is not justified. Supplemental imaging of the brain should be based on clinical findings or tissue diagnosis.