Eleven cats with papulocrustous dermatitis were randomly assigned to one of two groups and supplemented with either evening primrose oil or sunflower oil for 12 weeks. Clinical scores were assessed at intervals of three weeks and blood samples were taken for the analysis of plasma and erythrocyte fatty acid concentrations. The cats in both groups improved during the period of treatment and the concentration of linoleic acid in erythrocyte phospholipid increased in the cats fed evening primrose oil. Six weeks after the supplement was withdrawn the cats fed evening primrose oil had deteriorated less than those fed sunflower oil.