Background: Pelvis and trunk motions during baseball pitching are associated with ball velocity. Thus, limits in hip flexibility may adversely affect pitching biomechanics and the ability to generate ball velocity. Hypotheses: Professional baseball pitchers will have less passive range of motion in the nondominant hip and the measured ranges of motion of both the nondominant and dominant hips will correlate with biomechanical parameters of the lower extremity among professional pitchers. Study Design: Cross-sectional study; Level of evidence, 3. Methods: Nineteen healthy professional baseball pitchers volunteered for testing. Fluid goniometry was used to measure passive range of motion of adduction (ADD), abduction (ABD), internal rotation, external rotation, total arc of rotation, and total arc of ADD + ABD. Pitching biomechanical data were collected using an automated 3-dimensional motion analysis system while participants threw fastballs. Results: Pitchers possessed significantly less passive range of motion in the nondominant hip when compared with the dominant hip for all ranges. Total arc of rotation of the nondominant hip correlated with ball velocity (r =.50). Total arc of ADD + ABD in the nondominant hip and ABD in the nondominant hip were correlated with stride length (r = -.72 and.70, respectively). Dominant hip ABD (r =.63), total arc of rotation in the nondominant hip (r = -.45), and total arc of ADD + ABD of the dominant hip (r =.44) were correlated with trunk separation. Total arc of ADD + ABD of the nondominant hip (r = -.52) and total arc of rotation of the dominant hip (r = -.44) were correlated with pelvic orientation. Conclusion: Passive range of motion is smaller in the nondominant hip than the dominant hip among professional pitchers. The measured disparity between the hips is significantly correlated with various pitching biomechanical parameters of the trunk and pelvis. Future research is required to investigate a causal relationship between less hip passive range of motion and both ball velocity and pitching biomechanics. © 2010 The Author(s).