In a retrospective study to determine the anatomic nature of injuries in thumbs that were treated surgically for either fracture or instability, we reviewed 63 consecutive patients with acute skier's thumb injury. Of the 63 thumbs, 25 (40%) had a fracture. Surgical exploration showed 2 fracture types: a fragment that was attached to the ulnar collateral ligament, and a fragment that was not attached to the ulnar collateral ligament. The 1st type, corresponding to true avulsion fracture of the ulnar collaternal ligament, was found in 8 cases; the same fracture type was seen in another 7 cases, with an isolated fragment that was not attached to the ligament. Such an isolated fragment was observed in 10 other cases in which the ulnar collateral ligament was completely disrupted. This type of bony fragmentation cannot be differentiated from a bony avulsion of the ulnar collateral ligament on routine films. Therefore, stress testing the injured thumb is mandatory even when bony avulsion fracture with minimal displacement is suspected from a radiograph, as indeed the fracture may not be a bony avulsion but may be a fragmentation of the ulnar volar aspect of the proximal phalanx associated with a complete disruption of the ulnar collateral ligament.