Introduction:Glenohumeral internal rotation deficit (GIRD) is a well-documented finding in throwing athletes.Purpose:To investigate whether stretching the contralateral sacroiliac (SI) joint can improve GIRD in baseball players.Method:After internal shoulder rotation was measured in 23 minor league baseball players, the players randomly were assigned to either a control (ie, sleeper stretch of the dominant shoulder) or experimental (ie, SI joint stretch contralateral to the dominant shoulder) group. Afterward, internal rotation (IR) of their dominant shoulders was remeasured.Results:The mean initial end-range IR was 68.6° (SD = 7.9°) in the sleeper stretch group (n = 8) and 64.5° (SD = 5.1°) in the SI joint stretch group (n = 15). After stretching, the sleeper stretch group's mean end-range IR was 72.1° (SD = 7.2°), a 3.5° improvement (P = 0.1058), whereas the contralateral SI joint stretch group's mean end-range IR was 71.9° (SD = 6.6°), a 7.4° improvement (P = 0.0041).Conclusions:Stretching the contralateral SI joint improved GIRD more than the sleeper's stretch.