Renal sinus lipomatosis is the accumulation of excessive nontumorous fatty tissue within the renal sinus. In the normal young adult, a thin layer of loose fatty tissue envelops the pelvocalyceal and vascular structures which traverse the sinus. In the course of normal aging this layer undergoes a slight, gradual increase. Under extraordinary conditions the amount of fat in the renal sinus is so excessive that pelvocalyceal deformity results. The most obvious of these conditions is massive obesity. Proliferation of fat also occurs when there is loss of renal parenchyma as the result of infection, infarction, or arteriosclerotic ischemia. In a small number of cases lipomatosis occurs without evident cause. This entity has been referred to as fibrolipomatosis (11, 14), fatty replacement (3–6, 8, 10, 16, 18, 19, 23), fatty transformation (17), lipomatous paranephritis, and lipoma diffusum renis (8). Since neither replacement phenomena nor fibrosis are invariably present, we prefer the general term renal sinus...