The invention relates to a method for diagnosing mastitis in a lactating and preferably bovine animal, the method comprising recording data, the data having been obtained from analyzing a series of milk samples, the series of milk samples in turn having been obtained by periodically milking an animal, the data including total and/or differential leukocyte counts and changes in these count(s) between subsequent samples, wherein the differential leukocyte counts include the lymphocyte count, the macrophage count and the neutrophil count; and evaluating the recorded data to obtain information on the presence of mastitis and/or stage of mastitis and/or on the microorganism responsible for mastitis.