The ulnar collateral ligament (UCL) provides stability to the medial aspect of the elbow during valgus stress. Trauma to this ligament may result from repetitive forceful throwing. Diagnosis of UCL injury has been based on clinical findings of medial joint pain and valgus instability, as direct imaging of this structure has not been available. Eleven baseball pitchers with clinical evidence of UCL injury were evaluated with magnetic resonance (MR) imaging. Surgical correlation was obtained in six patients, four of whom underwent UCL reconstruction. MR imaging findings in UCL injury included laxity, irregularity, poor definition, and increased signal intensity within and adjacent to the UCL. These findings reflect the presence of hemorrhage and/or edema within the UCL due to repeated microtears, which eventually lead to weakening and possible disruption of the UCL. Optimization of spatial resolution, signal-to-noise ratio, and other technical factors is critical for evaluation of the UCL due to its small size. MR imaging is useful in documenting the presence and severity of injury to the UCL and in distinguishing this entity from other causes of elbow pain.