In this randomized, placebo-controlled, blinded field trial, 62 dogs (of which four were excluded) taken to a veterinary practice for orthopaedic surgery with a postoperative painful component were enrolled to assess the efficacy of a preoperative intramuscular injection of tolfenamic acid (TA) at a dose of 4 mg/kg in preventing postoperative pain. The animals were clinically examined at T1 + 1H, T1 + 4H, T1 + 24H (T1 = extubation). The efficacy results showed a statistical effect of TA in preventing postoperative pain with the evolution in the pain statistically in favour of TA treatment (Visual Analogue Scale). This was confirmed by the sum of the scores calculated at T1 + 24H that was statistically higher in the placebo group, and by the evolution in the respiratory rate, which was statistically lower in the TA-treated animals after surgery. TA treatment was very well tolerated as no clinical sign (except one isolated case of vomiting and diarrhoea, i.e. 3.5%) or change in biochemical and haematological values was observed and as no interaction with the anaesthetic drugs and with marbofloxacin was reported.