BACKGROUND: Despite much research, the reasons behind the development of chronic ankle instability in individuals post ankle inversion sprain are unknown. Chronic ankle instability has not previously been investigated dynamically using 3D motion analysis during walking. We hypothesised that chronic ankle instability subjects would exhibit a different kinematic and kinetic pattern during normal walking when compared with a control group. METHODS: Gait analysis was carried out on fifty subjects (25 chronic ankle instability, and 25 age, gender, activity, and gait velocity matched controls) during walking. Kinematic and kinetic pattern differences using the 3D motion analysis system combined with a force plate were established during 100 ms pre-heel strike and 200 ms post-heel strike, between the chronic ankle instability subjects and controls during normal walking. FINDINGS: Chronic ankle instability subjects were significantly (P<0.01) more inverted in the frontal plane compared to controls from 100 ms pre-heel strike to 200 ms post-heel strike. The joint angular velocity was significantly (P<0.05) higher at heel strike in the chronic ankle instability group. During the early stance phase of gait chronic ankle instability subjects appear to be controlled by an evertor muscle moment working concentrically compared to an invertor muscle moment working eccentrically in the controls. INTERPRETATION: These changes in kinematics and kinetics which arise are likely to result in increased stress being applied to ankle joint structures during the heel strike and loading response phases of the gait cycle. This could result in repeated injury and consequent damage to ankle joint structures.