Occupant dynamics during passenger vehicle underride has not been extensively evaluated. The present study examined the occupant data from IIHS rear underride crash tests. A total of 35 crash tests were evaluated. The tests were classified as full-width (n = 9), 50% overlap (n = 11), and 30% overlap (n = 15). A 2010 Chevrolet Malibu impacted the rear underride guard of a stationary trailer at 35 mph. Several occupant kinematics and dynamics data including head accelerations, head injury criteria, neck shear and axial forces, neck moments, neck indices, chest acceleration, chest displacement, chest viscous criterion, sternum deflection rate, and left/right femur forces/impulses, knee displacements, tibia axial forces, upper/lower tibia moments, upper/lower tibia indices, and foot accelerations were measured. The vehicle accelerations, delta-Vs, and occupant compartment intrusions were also evaluated. The results indicated that the head and neck injury parameters were positively correlated with driver A-pillar rearward intrusion. The 30% overlap crashes showed significantly higher intrusion and head and neck injury values than the 50% and full-width crashes. No strong relationship between head and neck injury parameters and vehicle delta-V or peak acceleration was observed. None of the chest injury criteria exceeded the chest IARV tolerances in the crash tests examined. No relationship between chest injury parameters and vehicle delta-V, acceleration or driver A-pillar rearward intrusion was observed. No strong relationship was observed between left/right leg injury parameters and vehicle delta-V, acceleration or driver A-pillar intrusion. Only for two crash tests, the “left upper tibia A-P moment”, “left upper tibia resultant moment” and “left upper tibia index” exceeded the IARV tolerances. This study suggested that in underride crashes there is a higher chance of head/neck injuries than other body regions. Also, in addition to delta-V, other parameters such as percent overlap and occupant compartment intrusion should be taken into consideration when analyzing the biomechanics of underride.