Valgus elbow stress leads to medial tension and lateral compression injuries in baseball pitchers of all ages. This study was undertaken to investigate the relationship between elbow stress in professional baseball pitchers and the kinematic parameters of pitching mechanics. This was done in an attempt to understand valgus extension overload better and in an effort to improve preventive and rehabilitative protocols. High-speed video data were collected on 40 professional pitchers in game situations during the 1998 and 1999 Cactus League season in Arizona, as part of Major League Baseball Spring Training. A multiple linear regression analysis was used to relate elbow valgus to kinematic parameters of pitching mechanics. The resulting analysis produced an adjusted multiple R2 value of 0.974, indicating that nearly 100% of the variance in valgus stress on the elbow was explained by the parameters in the regression equation. This ability to explain over 97% of the variance in valgus stress is significant. The parameters of pitching mechanics related to elbow valgus may be assessed and optimized, if necessary, in order to decrease the magnitude of elbow stress in pitching.