The postural stability of wheelchair users experiencing external perturbations was examined. Rotation of a tilt platform generated moments in the trunks of subjects seated in a manual wheelchair on the platform. The magnitude and duration of the moments were on the order of those that might be encountered in the sagittal plane during controlled braking maneuvers in a vehicle. Four subjects with tetraplegia, four with paraplegia, and five controls participated in experimental trials on the platform. As input, four different Disturbance profiles with either a 0.2 g (gravitational acceleration) or 0.4 g maximal level were imposed. The majority of the subjects with spinal cord injury lost balance at Disturbance levels below 0.2 g. The results suggest that the rate of change of the applied perturbation may also affect stability. The use of a stability index based on normalized motion of the center of pressure with respect to the seat showed efficacy in characterizing the response.