Four patients with idiopathic retroperitoneal fibrosis were found to have characteristic obstruction and anterior displacement of the lumbar vena cava. Varying degrees of venous collateral circulation were present, depending on the degree of vena caval compression. All 4 patients underwent ureterolysis to relieve the hydronephrosis and to confirm diagnosis by biopsy. Two patients were treated with corticosteroids postoperatively. Followup venacavography demonstrated improved filling and decrease in the collateral circulation in these 2 patients. Of the remaining 2 untreated patients 1 was lost to followup and the other revealed no change. Diagnosis of idiopathic retroperitoneal fibrosis can be made if characteristic changes are present on inferior venacavography and may provide an objective measure for following the basic disease process during medical treatment.