We conducted a study to evaluate the effect of platelet-rich plasma (PRP) injections on partial ulnar collateral ligament (UCL) tears in high-level throwing athletes. We retrospectively reviewed the cases of 44 baseball players (6 professional, 14 college, 24 high school) treated with PRP injections for partial-thickness UCL tears. All tears were diagnosed by physical examination and confirmed by magnetic resonance imaging (MRI). Sixteen patients had 1 injection, 6 had 2, and 22 had 3. Once patients became asymptomatic after injection, they were started on an interval throwing program. Physical examination findings at final follow-up were classified according to a modified version of the Conway Scale. Mean age was 17.3 years (range, 16-28 years). All patients were available for follow-up after injection (mean, 11 months). Of the 44 patients, 15 (34%) had an excellent outcome, 17 had a good outcome, 2 had a fair outcome, and 10 had a poor outcome. After injection, 4 (67%) of the 6 professional players returned to professional play. Twenty-two patients had proximally based partial-thickness tears, 7 had distally based partial tears, and 15 had diffuse signal without partial tear on MRI. Mean time from injection to return to throwing was 5 weeks; mean time to return to competition was 12 weeks (range, 5-24 weeks). There were no injection-related complications. Our use of PRP in the treatment of UCL insufficiency produced outcomes much better than earlier reported outcomes of conservative treatment of these injuries. PRP injections may be particularly beneficial in young athletes who have sustained acute damage to an isolated part of the ligament and in athletes unwilling or unable to undergo the extended rehabilitation required after surgical reconstruction of the ligament.