['Elite throwing athletes have increased proximal humeral retrotorsion (HRT) and glenoid retroversion (GRV) in their throwing shoulders compared with their nonthrowing shoulders. These adaptive morphologic changes are thought to be independently protective against shoulder injury; however, their relationship to each other is poorly understood.', 'To determine if an association exists between HRT and GRV within the same shoulders of professional pitchers.', 'Cross-sectional study; Level of evidence, 3.', 'The HRT and GRV measurements were determined using published techniques in asymptomatic bilateral shoulders of 32 professional pitchers (mean age, 23 years). Three measurements for each variable were averaged, and the reliability of the techniques was verified. The relationship between HRT and GRV within the same shoulders was determined with Pearson correlation coefficients. Paired t tests were used to compare HRT and GRV between the throwing and nonthrowing shoulder. Simple ratios were calculated between HRT and GRV.', 'Humeral retrotorsion and GRV were both significantly greater on the throwing side compared with the nonthrowing side (HRT: throwing = 9.0° ± 11.4° and nonthrowing = 22.1° ± 10.7°, P < .001; GRV: throwing = 8.6° ± 6.0° and nonthrowing = 4.9° ± 4.8°, P = .001). Within the same shoulders, there was a statistically significant positive association between HRT and GRV on the throwing side (r = 0.43, P = .016) but not on the nonthrowing side (r = -0.13, P = .50). The HRT:GRV ratio was 2.3:1 for throwing shoulders and 7:1 for nonthrowing shoulders.'] Do bony adaptation of the proximal humerus and glenoid correlate within the throwing shoulder of professional baseball pitchers?