Diagnostic ultrasound studies were performed in 104 patients with renal fibrolipomatosis. The lesions were echo-poor in 94 of 104 cases and echogenic in the remaining 10 patients. The echo-poor lesions presented multiple echo-poor zones, a single echo-poor region of a finger-shaped configuration. The latter pattern resembled hydronephrosis. Differentiation between the two lesions was possible on the basis blurred contours, absence of increased transmission and lower echo intensity in fibrolipomatosis than in urinary tract obstruction. Cases of echogenic fibrolipomatosis are characterized by a high proportion of collagen fibres. Differential diagnosis of these lesions including tumours of the renal parenchyma and pelvis are discussed.