The presence and degree of tarsocrural joint effusion and lameness, and the result of a hindlimb flexion test were scored subjectively in 79 horses before, and six weeks to 20 months after, arthroscopic surgery to remove osteochondritis dissecans fragments. The scores of all three variables improved significantly after surgery. The score reductions for the right and left hindlimbs, respectively, were 82 per cent and 95 per cent for lameness, 48 per cent and 41 per cent for joint effusion, and 89 per cent and 84 per cent for reaction to flexion (P≤0·01). The oldest horses reacted more favourably to the operation, as measured by the reaction to the flexion test, but age was not significantly related to changes in lameness or joint effusion. There was no significant correlation between the time of follow-up examination and the effect of surgery on lameness and reaction to flexion, but an increased time to follow-up was associated with decreased joint effusion.