The application of adhesive tape to the ankle prevents sprains and is therefore frequently used by athletes with functional instability of that joint. The purpose of the study was to investigate the effect of taping on postural sway during a sudden perturbation in subjects with unilateral functional instability of the ankle joint. Nine Swedish male elite soccer players with unilateral functional instability of the ankle joint after a sprain participated in the study. Their mean age was 25 years. All but one had sustained an ankle sprain during the last 1-12 months, and they applied adhesive tape on one ankle at every practice session and game. Eight moderately active male subjects served as controls (mean age 32 years). Recording and analysis of postural sway was performed by the use of stabilometry. We used a device previously tested for reliability and added a perturbation device. The perturbation device was an ankle disc standing on four bars on top of the force plate. At the start of each recording, the ankle disc was stable. The bars could be removed simultaneously by the test leader. The following parameters were analysed: (1) max sway, the maximal sway amplitude upon perturbation in mediolateral and anteroposterior directions; (2) mean sway, the mean sway during the recording in mediolateral and anteroposterior directions. There was no difference between the right and left ankles before the practice session. The postural sway during perturbation (max sway) decreased when the ankle was taped before the exercise. After the exercise there was no difference between the taped and untaped ankles. The maximum and mean amplitude of sway without tape were smaller after exercise than before exercise. The results in the present study indicate a positive effect of tape on postural sway during perturbation before a practice session. The decreased sway with an untaped ankle after the practice session could mean that the prophylactic effect of taping is most important during the first part of a practice session or a game. Warmer muscle units mean less uncontrolled muscle activity which could result in more effective stabilisation of the ankle joint.