Congenital unilateral upper limb muscular hypertrophy is a rather new entity. Patients have unbalanced muscular forces on their upper extremities due to muscular hypertrophy. These unbalanced forces preclude physiological movements, and in the late course, cause joint deformities. Once joint deformities occur, benefit from surgery diminishes; botulinum toxin can be used for the prediction of a possible benefit from surgery. A 14-year-old girl presented with flexion and ulnar deviation contracture of the left wrist, narrowing of the first web space, flexion contracture of all fingers, and more evident on the first, third, and fourth fingers. After botulinum toxin injection, the most notable benefit was seen on the wrist contracture. In surgery, adductor myotomy, flexor carpi ulnaris transfer to palmar aponeurosis, and tendon transfers of flexor digitorum superficialis to flexor digitorum profundus in the third, fourth, and fifth fingers were made. The postoperative gain was similar to the outcome after botulinum toxin injection. Botulinum toxin may help the surgeon to understand if chronic joint deformities have occurred and predict possible benefit from surgery.