Four patients presented with severe renal failure secondary to urinary tract obstruction, yet ultrasonography and/or computed tomography revealed only minimal dilatation in 1 patient and no dilatation in the other three. Two patients had prostate cancer, one had bladder cancer, and one had retroperitoneal fibrosis. In all cases, relief of obstruction led to a dramatic improvement in renal function. These cases, and others in the literature, illustrate that in certain settings severe urinary tract obstruction may be present in the absence of dilatation and hence may be missed by noninvasive imaging techniques. Nondilated obstructive uropathy should be suspected in any elderly patient who presents with the acute onset of oliguria in the absence of an identifiable cause, especially if there is a previous history of malignancy in the pelvis. Left undiagnosed, this potentially reversible cause of renal failure can lead to end-stage renal disease.