To analyze the effect of inversion trauma on peroneal nerve function, motor conduction velocity was measured in 22 patients. In the injured leg, 4–8 d post trauma motor nerve conduction velocity in the knee-caput fibulae segment of the superficial peroneal nerve was significantly smaller when compared with the contralateral leg and the control group. Five weeks post trauma these values were normal again. For three segments of the deep peroneal nerve, the motor conduction velocity was significantly reduced, 4–8 d post trauma, when compared with the control group. In the caput ankle and knee-ankle segment, motor conduction velocity was still significantly lowered 5 wk post trauma. Lowered amplitudes of the Compound Motor Action Potentials of the extensor digitorum brevis muscle were found 4–8 d post trauma. No correlation was found between motor nerve conduction velocities and subjective clinical tests (anterior drawer sign and (manually performed) talar tilt test). The results of this study support the hypothesis that inversion trauma is frequently accompanied by lesions of the peroneal nerve. Motor conduction velocity measurements can be a valuable tool in assessing more objectively functional instability of the ankle joint induced by inversion trauma.