The efficacy of ketoprofen in the treatment of acute clinical mastitis was evaluated in a clinical trial comprising a non-blind controlled study and a blind, placebo-controlled study. All the cows were treated with 20 g sulphadiazine and 4 g trimethoprim intramuscularly upon diagnosis, and half the dosage was given once daily thereafter. In addition, the ketoprofen treatment groups received 2 g ketoprofen intramuscularly once daily for the duration of the antimicrobial therapy. Recovery rates for the non-blind contemporary controls and the blind placebo-controls were 83·7 per cent and 70·7 per cent, respectively. In the non-blind controlled ketoprofen and the placebo-controlled ketoprofen treatment groups, recovery rates were 94·7 per cent and 92·3 per cent, respectively. The odds ratio (OR) of recovery was significantly (P<-0·01) high in the placebo-controlled study (OR = 6·75, confidence interval [CI] = 1·45 to 31·4), and high but not significant in the non-blind controlled study (OR = 2·64, CI = 0·53 to 13·10). It was concluded that ketoprofen significantly improved recovery in clinical mastitis in dairy cows.