The gamekeeper's or skier's thumb is a very common injury. Nondisplaced tears of the ulnar collateral ligament of the thumb (UCL) may be treated conservatively. For that reason an accurate diagnosis is mandatory for choosing the correct therapy. Ultrasound is able to depict the position of the torn UCL correctly in approximately 90 % of cases. Sonographic pitfalls can be caused by a dislocation of the palmar joint capsule to the ulnar joint space, by a scalloping of the adductor aponeurosis due to the displaced UCL, and by scar tissue or technical mistakes. It is important to know about those pitfalls because conservative treatment of displaced UCL tears leads to instability. Therefore, the use of MRI is recommended whenever a nondisplaced UCL tear is suspected by US and a conservative therapy is suggested. Splitting the diagnostic pathway between US and MRI and preferring conservative therapy in nondisplaced UCL tears should help to save money in this field.