The differential diagnosis of acute flank pain includes kidney stones, urinary tract infection, dissecting abdominal aortic aneurysm (AAA), arterial or venous compromise of the kidney, renal abscess, renal carcinoma, and papillary necrosis. This is a report of an unusual cause of renal colic: pyelocalyceal diverticulum. Stasis of urine within a diverticulum promotes both calculus formation and urinary tract infection, either of which can lead to colic. Several radiographic findings may suggest a calyceal diverticulum, including, on plain X-ray, a very peripheral or mobile renal calculus, or on intravenous pyelogram, an early filling-defect and delayed or retained filing of a circular or ovoid mass.