The pain-relieving effect of carprofen and tolerance to the drug were investigated in 805 dogs that were lame as a result of osteoarthritis. The dogs were of different breeds, ages and bodyweights and of both sexes, and were selected from 51 veterinary clinics. Each dog was treated orally by its owner with 4 mg/kg carprofen for 84 consecutive days. Twenty-four dogs were removed from the study because of side effects, and 55 left the study for reasons unrelated to the treatment. The condition of the dogs and the benefit of the treatment were evaluated by the veterinary surgeons and the owners after 14 days, and at the end of the period of treatment, when 194 of the dogs (26·7 per cent) were no longer lame, and 357 (49·2 per cent) had improved. The period for which the dogs had been lame before entering the study significantly (P<0·01) affected the results and the rate of improvement. Too much exercise during the 84 days of treatment caused some dogs to relapse.