Montly investigations have been carried out on a farm for the presence of subclinical mastitis in a total of 59 cows. As many as six examinations have been made up to the drying of the cows and two after their calving. It has been found that subclinical mastitis assumes a protracted course, and in 23.7 per cent of the cows it is present during the whole period of investigation of 6-8 months. In 42.9 per cent of the animals the recovery of the affected udder sets in without therapeutic intervention. It has also been noted that in 33.8 per cent of the diseased cows subclinical mastitis develops into a clinical form of mastitis.