Two adult patients with acute renal vascular occlusion with infarction are described. Both patients were believed to have ureteral colic. In each instance, the correct diagnosis was overlooked at the initial emergency department visit. An uncommon clinical entity that continues to go undiagnosed, acute vascular occlusion of the kidney must be considered in the differential diagnosis of acute flank pain. Absence of the nephrogram phase on an intravenous pyelogram (IVP) should alert emergency physicians to this possible diagnosis and to the need for further work-up. Subsequent diagnostic evaluation should begin with renal ultrasonography to rule out obstructive uropathy. If hydroureteronephrosis is not present, follow-up perfusion studies are necessary to confirm the absence of renal perfusion. Greater awareness of this uncommon clinical entity and its potential morbidity is essential to correct diagnosis and management.