PURPOSE: This study aimed to analyze differences in breast kinematics between breast cup sizes during running and the ability of breast and body size measurements to explain these differences. METHODS: Forty-eight women (A to G cup; mean ± SD: age = 26.0 ± 6.0 yr, stature = 1.667 ± 0.064 m, mass = 62.78 ± 8.24 kg) with chest sizes of 32 to 38 inches participated. Chest and breast girths, a restricted anthropometric profile, suprasternal notch to nipple distances, and body mass index were measured, and breast mass was estimated. Multiplanar relative breast displacement, velocity, and acceleration during treadmill running were then recorded. Differences in breast kinematics were compared between cup sizes before and after allometric/polynomial scaling using significant breast and body size measures. RESULTS: All kinematic variables significantly increased with breast cup size (P < 0.05). Mean anterior-posterior (a/p), medial-lateral (m/l), and vertical bare-breasted displacements ranged from 0.030 to 0.059 m, from 0.018 to 0.062 m, and from 0.042 to 0.099 m, respectively, across A to G cups. Breast velocities ranged from 0.428 to 1.244 m·s (a/p), 0.411 to 1.708 m·s(m/l), and 0.819 to 2.174 m·s (vertical), respectively. Increases in breast acceleration varied from 11.664 to 48.438 m·s (a/p), 15.572 to 51.987 m·s (m/l), and 23.301 to 66.447 m·s (vertical), respectively. Scaling models found that breast mass was the only anthropometric measure to consistently explain differences in breast kinematics between cup sizes. CONCLUSIONS: Bare-breasted kinematics significantly increased with cup size during running. Differences in breast displacement, velocity, and acceleration between cup sizes could be predicted using estimates of breast mass based on conventional brassiere sizing. These data inform the design and evaluation of effective bra support.