Anecdotal reports suggest two-step star jumps cause excessive breast movement and discomfort, leading to recommendations for this activity as a diagnostic tool to determine effective breast support in a retail environment. The aim was to investigate multiplanar bare-breast kinematics during the two-step star jump and to establish the relationship between breast kinematics, discomfort and cup size. Thirty-nine females completed five two-step star jumps with no breast support after which breast discomfort was rated. To establish relative breast kinematics infrared cameras tracked the 3D co-ordinates of breast and body markers. Maximum resultant breast displacement, velocity and acceleration during jumping reached 18.7 cm, 93.1 cm·s⁻¹ and 3.6 g, respectively. Significantly more vertical breast displacement (p < 0.01) and velocity (p < 0.01) occurred compared with mediolateral and anteroposterior kinematics. Breast discomfort increased as cup size increased (r = .61). Two-step star jumping stimulated multiplanar breast kinematics and high levels of breast discomfort. Therefore, this activity may be useful in a retail outlet to determine the function and comfort of a sports bra.