Context: Glenohumeral (GH) internal-rotation deficit (GIRD) and lower eccentric external-rotator (ER) to concentric internal-rotator (IR) strength (ER:IR) ratio have been documented as risk factors for shoulder injuries, but there is no information on whether GIRD has an adverse effect on ER:IR ratio in adolescent overhead athletes. Objectives: The aim of this study was to investigate the effects of GIRD on functional ER:IR ratio of the adolescent overhead athletes. Design: Cross-sectional study. Setting: University research laboratory. Participants: 52 adolescent overhead athletes. Main Outcome Measures: To determine GIRD, the range of GH IR and ER motion was measured with a digital inclinometer. An isokinetic dynamometer was used to assess eccentric and concentric IR and ER muscle strength of the dominant and nondominant shoulders. One-way ANCOVA where sport type was set as a covariate was used to analyze the difference between athletes with and without GIRD. Results: After standardized examinations of all shdulders, the athletes were divided into 2 groups, shoulders with (n = 27) and without GIRD (n = 25). There was a significant difference between groups in functional ER:IR ratio (P <.001). Athletes with GIRD had lower ER:IR ratio (0.56) than athletes without GIRD (0.83). Conclusions: As GIRD has an adverse effect on functional ratio of the shoulder-rotator muscles, interventions for adolescent overhead athletes should include improving GH-rotation range of motion.