PURPOSE: To assess passive range of motion (ROM) measurements acutely over time and how this acute change alters over the course of a baseball season. METHODS: Seven healthy male NCAA Division I baseball pitchers were measured prior to the start of the season. Each pitcher was assessed for passive shoulder and elbow ROM, with measurements taken after each pitching bout during the season. Pitching bouts were allotted to early season, mid-season, or late season. In addition to ROM measurements, pitch volume, pitch type, and ball velocity were also collected for analysis. RESULTS: Following a pitching bout, passive shoulder external rotation decreased by 7% (p = 0.000) during the early season. However, passive shoulder internal rotation was significantly reduced during the early season (-37%; p = 0.001), mid-season (-30%; p = 0.000), and late season (-42%; p= 0.000). Further, total motion also decreased during the early season (-12%; p= 0.000), mid-season (-6%; p= 0.000), and late season (-9%; p= 0.000). In terms of pitch type, increasing the volume of fastballs thrown had a moderate correlation with loss of passive shoulder external rotation during the late season (r= -0.705). Increasing the volume of curveballs thrown had a moderate correlation with total motion lost during the late season (r= -0.665) as well as with passive elbow extension lost during mid-season (r= -0.760). CONCLUSIONS: Passive ROM is significantly impacted immediately after throwing. This decrease continues to be present throughout all parts of the baseball season. Moreover, a moderate correlation of fastballs and curveballs to losses in passive ROM indicates that attention to pitch type may be warranted, particularly during the middle and latter parts of the baseball season.