Chronic lateral instability of the ankle, a condition with frequent ankle sprain episodes, difficulties to walk and run on uneven ground and often pain, occurs in 20-30% after acute ankle sprains. The treatment of CLI as well as of acute ankle sprains have more and more in addition to operative reconstructions been focused on proprioceptive training which is in concordance with the results of recent research results. The present study shows that the reaction time after sudden angular displacement of the ankle on a trapdoor is increased by approximately 15 ms in patients with chronic lateral instability compared to control persons. We conclude that delayed proprioceptive response to sudden angular displacement of the ankle can be one of the causes to chronic lateral instability of the ankle and that proprioceptive training should be included in the treatment of acute and chronic ankle disabilities.