Bilateral hydrocalycosis secondary to radiographically demonstrated bilateral infundibular stenosis required a left nephrectomy in an 18-year-old young man. The infundibular narrowing was due to marked hyperplasia of the submucosal smooth muscle. Scattered within the hypertrophic smooth muscle were rare spindled CD117-positive interstitial cells, similar to the cells responsible for peristaltic pacemaker activity within the gastrointestinal tract. The admixture of CD117 cells suggests this is a true muscular hyperplasia rather than a leiomyomatosis. The stimulus to the hyperplasia is unknown.