In this Research Communication we report the results of a controlled study conducted under field conditions in which we analysed milk ejection curves in cows with chronic mastitis, and assessed the influence of antinflammatory treatment with ketoprofen. Total milking time was reduced in chronic mastitis cows, irrespective of ketoprofen treatment, and the proportion of bimodal flow curves was increased. This latter effect was partially reversed by ketoprofen. To our knowledge, this is the first study showing that chronic mastitis has a significant effect on the milk ejection curve. Antinflammatory treatment with ketoprofen was shown to be efficacious in reducing these negative effects, re-establishing a pattern close to the one observed in healthy cows.