Injuries around the tarsal joint are common in dairy cows kept in tie stalls. This study investigated the influence of the frequency and duration of outdoor exercise on the prevalence of hock lesions in tied Swiss dairy cows. Over a 1-year period (from January to December), cows on 66 farms were examined a total of six times (once every 2 months) for the number and severity of hock lesions (hairless patches, scabs and open wounds, swellings). The prevalence of scabs and wounds (mean 2.1 scabs per cow and farm, range 0.3-4.1) was negatively associated with the duration of outdoor exercise, and positively associated with its frequency. It was also significantly affected by the time of visit and the type of bedding (straw being better than other materials). With increasing length of the lying area, the prevalence of scabs and wounds decreased. Based on the interpretation of the final model, it is suggested that a minimum of 50h spent outdoors over a 4-week period is necessary to have a marked result on the prevalence of hock lesions.