Background: While the high rate of ulnar collateral ligament (UCL) injuries in professional baseball is widely discussed in the media and medical literature, the actual prevalence of UCL reconstruction has not been documented. Hypothesis: The prevalence of UCL reconstruction will be higher among pitchers than nonpitchers, and Major League Baseball (MLB) pitchers will have a higher prevalence than will minor league pitchers. Study Design: Descriptive epidemiology study. Methods: An online questionnaire was distributed to all 30 MLB organizations. Certified athletic trainers from each team administered the questionnaire to all players in the organization, including major league players and 6 levels of minor league players. Demographic data were compared between major and minor league players. Continuous variables (age, years of professional baseball, country of origin, etc) were compared with Student t tests (P<.05). Categorical variables (level, position, etc) were compared using chi-square analysis (P<.05). Results: A total of 5088 professional baseball players (722 major league and 4366 minor league) participated in the survey. Pitchers represented 53% of all players, and 497 players reported at least 1 UCL reconstruction, demonstrating a prevalence rate of 10% (497 of 5088). Pitchers reported a significantly higher prevalence of UCL reconstruction (16%; 437 of 2706) than nonpitchers (3%; 60 of 2382; P<.001). Among major league pitchers, 25% (96 of 382) had a history of UCL reconstruction, while minor league pitchers showed a 15% (341 of 2324) prevalence (P<.001). Major league pitchers were also significantly older (28.8 ± 3.9 years) than minor league pitchers (22.8 ± 3.0; P<.001). The majority of major leaguers (86%) had their UCL reconstruction as professional pitchers, whereas the majority of minor league pitchers (61%) underwent their UCL reconstruction during high school and college (P < .001). The rates of UCL revision, prior elbow surgery, prior shoulder surgery, and types of UCL graft were similar between the major league and minor league pitchers. No difference in prevalence was shown between pitchers born in the United States versus Latin American countries. Conclusion: Pitchers have a high prevalence of UCL reconstruction in professional baseball, with 25% of major league pitchers and 15% of minor league pitchers having a history of the surgery.