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Stress fractures of the first rib are uncommon, and few reports have described the occurrence of this injury in overhead-throwing athletes. Furthermore, although there have been a few reports of first rib stress fractures on the throwing side in baseball players, fractures on the nonthrowing side are very rare. Here, we report 2 cases of first rib stress fractures on the nonthrowing side related to swinging of a baseball bat. The cause of the fractures in the present cases may have been repetitive traction and shear force on the first rib resulting from muscle exertion while swinging a bat. Conservative treatment that considered the mechanism of stress fractures resulted in a symptom-free and complication-free return to baseball. The patient's background should be considered for an accurate understanding of the injury mechanism, adequate conservative therapeutic plan, and a successful return to baseball.