We report 2 cases of spontaneous thrombotic occlusion of the main renal vessels presenting with acute lumbar flank pain and hematuria suspect of nephrolithiasis. Clinical and laboratory signs of blood hypercoagulability, generalized arterial embolism, nephrotic syndrome or glomerulonephritis were absent. Excretory urography, nephrosonography and retrograde ureteropyelography showed no evidence of upper urinary tract calculi or other causes of obstruction. Renal angiography and cavography demonstrated an acute renal vein thrombosis in 1 patient and a thrombotic occlusion of all but one of the segmental renal arteries in the other patient. These 2 cases demonstrate that thrombotic occlusion of the renal artery or renal vein has to be considered in patients who are presenting with lumbar flank pain and hematuria, in whom the excretory urogram shows severe malfunction of one of the kidneys, and stone disease can be excluded. Renal angiography and cavography as well as CT scan should be carried out in these patients. When the disease is diagnosed at an early stage, an intra-arterial thrombolysis can be attempted.