A 16-year-old boy was treated by a radial-shortening procedure for symptomatic stage IIIB Kienböck's disease with 4 mm negative ulnar variance. The osteotomy corrected the ulnar variance to negative 1 mm after surgery, but further negative ulnar variance of 9 mm occurred at follow-up evaluation as a result of radial overgrowth. The functional outcome was excellent with remodeling of the lunate. The possibility of overgrowth should be considered when contemplating a radial-shortening osteotomy for Kienböck's disease in skeletally immature patients.