Baseball players frequently injure their shoulders and elbows. Preseason risk factors for such injuries have been relatively well studied, but in-season risk factors are less known, and the relationship between the number of pitches and the incidence of such injuries in baseball pitchers of any level is unclear. Identifying the risk factors related to the number of daily pitches is particularly important to prevent baseball-related arm injuries among young pitchers. Thus, we prospectively investigated the relationship between the number of daily full-power pitches in high school baseball pitchers and the incidence of shoulder and elbow injuries. We observed that a small number of daily full-power pitches, < 30 pitches per day, in high school baseball pitchers, significantly increased the risk of shoulder and elbow injuries: these players had a 2.3-times greater risk of injuries and a 45-day earlier occurrence of injuries than those pitching ≥ 30 pitches per day. Although unexpected, this was plausible as continuous daily pitching is required to maintain physical condition in growing and maturing high school pitchers. These findings may form the basis for establishing guidelines regarding the appropriate number of daily pitches required to prevent shoulder and elbow injuries in high school baseball pitchers.