The aim of the paper was to discuss the factors determining the susceptibility of cows to mastitis, the most important for breeders, including losses incurred by milk producers due to the disease. Inflammations of mammary gland are common and the most awkward problem which dairy farmers face around the world. In the case of clinical udder inflammation, it is estimated that the cost of mastitis is about 200 € per cow with fluctuations from 100 even up to 1000 €. Resistance, i.e. the susceptibility of cows to mastitis, is conditioned genetically. The possibility of BoLA and lactoferrin gene polymorphism, using in the selection cows resistant to mastitis, is indicated. Bacteria inhabiting the mammary gland, and the living environment of animals are primarily the main etiological factor. Moreover, many authors indicate the differences between breeds of cows in resistance to mastitis. High-productive breeds are generally more susceptible to the udder inflammation, in comparison to the local breeds. The greatest reduction in milk yield is observed in the case of these breeds, and especially in Holstein-Friesian breed, in which the decline of daily yield at somatic cell count (SCC) amounting above 500,000 per ml of milk can reach 25%. The probability of mastitis occurrence increases with an age of cows and following lactation, and dry period is a time of increased risk. Higher incidence of inflammation is also associated with abnormalities in the construction of the udder (unevenly developed quarters, too long or too short teats). In recent years, a possible relationship between feeding system of cows with their resistance to udder infections has been reported. It is emphasized that a deficiency of selenium and vitamins A and E in the ration may contribute to the increase in the number of mastitis incidences. There are, however, unclear opinions of various authors on the effect of automatic milking system on udder health.