Injuries to the ulnar collateral ligament of the metacarpophalangeal (MCP) joint of the thumb are relatively common. When an incomplete rupture is present, valgus stress testing with the MCP joint positioned in extension reveals minimal or no instability (less than 30 degrees of laxity or less than 15 degrees more laxity than in the noninjured thumb). When a complete rupture is present, valgus stress testing with the MCP joint positioned in extension reveals marked laxity (more than 30 degrees or more than 15 degrees more laxity than in the noninjured thumb). In this instance, displacement of the ligament proximal and superficial to the adductor aponeurosis, which is often termed a Stener lesion, is likely. Partial ligament injuries in which the ligament is not displaced may be treated nonoperatively. When a Stener lesion is present, however, primary ligament healing cannot occur without operative management. Whether treatment should be operative or nonperative can generally be decided on the basis of the findings from the history, the radiographs, and the physical examination, which should include valgus stress testing.