The radiological appearances of 11 cystic renal cell carcinomas (RCC) presenting over a 13-year-period are reviewed and correlated with their clinicopathological features. The tumours occurred in seven males and four females with a mean age of 59.1 years (range 27-87 years). Their ultrasound features were categorized according to Bosniak [5]. There were six category 3 or 4 cysts (indeterminate or malignant); three category 2 cysts and two category 1 cysts. Computed tomography scans (CT) demonstrated malignant features in all seven patients scanned. Cyst aspiration, intravenous urograms and arteriograms were generally unhelpful. Histological growth patterns of nine tumours were determined and all were staged by Robson's classification: eight were Stage 1, one was Stage 3 and two were Stage 4. Three of four deaths were related to tumour bulk, and the fourth patient died from an unrelated cause. All seven survivors had Stage 1 disease. No correlation existed between different histological subtypes and tumour aggression. Overall, the behaviour of cystic RCCs was similar to those of solid RCCs apart from the high percentage (72.2%) of Stage 1 disease. We conclude that any deviation from a simple cyst on ultrasound should be assessed by CT and/or surgery. Indeterminate lesions on CT including multilocular lesions should be surgically explored.