Proper treatment for injuries to the ulnar collateral ligament (UCL) of the metacarpophalangeal joint of the thumb relies on an accurate diagnosis of the pathology of the ligament. The current study evaluates the accuracy of ultrasonography to diagnose the specific pathology related to UCL injuries. The charts of 16 patients who underwent 21 studies of the UCL were reviewed. Ten patients had studies that indicated a complete tear, and eight of those patients underwent surgical exploration. Intraoperatively, 7 of the 8 patients were found to have ruptured ligaments. Six patients had studies that indicated intact ligaments. Four patients were treated with immobilization and a fifth patient underwent surgical treatment for a separate pathology. All patients regained stability of the ligament. The final patient had a repeat examination that indicated a rupture and the patient was found to have a markedly attenuated ligament on exploration. In conclusion, ultrasound is a useful modality for making a correct diagnosis of UCL pathology.