The objective of the present study was to examine the relative timing of pelvis and thorax rotations in achieving high throwing velocities in baseball pitching. During the preseason, a kinematic analysis was performed on eight pitchers. Peak angular velocities of the pelvis and thorax were determined and separation, defined as the time between the moments of maximal rotation velocity of the pelvis and thorax, was calculated. By themselves, maximal pelvis and thorax rotation velocity were not associated with throwing velocity. Separation was positively and significantly associated with throwing velocity. Results indicate that the relative timing of pelvis and thorax peak rotation velocity in pitching fastballs in baseball is a determinant of throwing velocity in skilled pitchers.