With the availability of modern angiographic technics, it has become a simple matter to establish the diagnosis of thrombosis of the renal veins. It is a more difficult problem, however, to suspect this condition on the basis of simpler roentgenographic procedures and to refer suitable candidates for definitive angiographic study. Clinically, patients present with the nephrotic syndrome and almost invariably with flank pain, resulting from capsular stretching by the engorged kidney (3, 4, 15). Painful congestion of the corresponding testicle has been described (22) and was present in one of our patients. Hematuria is common (15, 23). During the past several years we have studied six patients with renal vein thrombosis (Table I). It is our purpose to demonstrate those roentgen findings which we have found to be helpful. Roentgen Observations Plain film of the Abdomen: Initially, the renal outlines will be enlarged, often massively-we have encountered craniocaudad measurements of 19 cm. Figure 1, A, demonstrates the right kidney of Case IV to be enlarged shortly after onset of symptoms in October of 1960. By December of 1960, the corresponding measurements were 10 × 5 cm (Fig. 1, B), and the kidney remained nonfunctioning. The disease was unilateral and the patient survived. This course of events follows remarkably closely that found by Koehler et al. (17) after experimental renal vein occlusion in dogs. They demonstrated that kidney size increased rapidly during the first twenty-four hours, peaking at one week, and then diminished over a two-month period, ending in atrophy. Such unilateral thrombosis has been shown to result in the nephrotic syndrome (20). In our experience, the condition is more frequently bilateral (Cases I–III, V, and VI). These patients survive by virtue of collateral venous drainage (8). It has been suggested (7) that gradual occlusion is a prerequisite for the formation of collaterals and survival. As collateral drainage develops, the initially enlarged kidney diminishes in size but remains larger than normal. The nephrotic syndrome also usually persists to some degree. In those unusual cases in which the occlusion spontaneously resolves (Case I), renal size returns to normal and the nephrotic syndrome relents. Excretory Urography: If occlusion is complete and no collateralization forms, there will be no visualization of the collecting systems on excretory urography. In all other cases, there is initial renal enlargement and opacification of varying degree, always delayed and diminished but often surprisingly dense (15, 23) (Fig. 2). When the infundibula and pelvis are well visualized, they can be seen to be stretched, compressed, and often virtually occluded by interstitial edema and swollen renal parenchyma (Fig. 3). This appearance has been compared to that seen in polycystic disease (4, 9, 23, 25) and has on occasion led to this mistaken diagnosis (24).