The frequency of mastitis in more than 28 thousand cows was investigated. We report on how age and level of productivity, season of the year, "milkmaid" factor, udder properties, and hereditary factors influence mastitis frequency. The influence of the mothers and fathers on resistance or predisposition of the daughters to mastitis is shown. Among daughters from sick mothers the disease is two and more times higher than in the offspring of healthy cows; the frequency of mastitis ranges among families from 0 up to 57%. High variability of the sire's breeding value on mastitis stability (from -4.2 up to +24.0%) was found established. The factors of mastitis inheritance are low, on average 0.1. It is possible to estimate the bull's effect on resistance to mastitis in first-calving cows if a significantly large group is available. Among the bulls assessed in mastitis, sires were found to be plus proven (29.6%), neutral (44.4%) and poor proven (26.0%) in relation to the resistance parameter. The ranks of the bulls on mastitis frequency in the daughters and on sire's breeding value are characterized by high reproducibility (0.7 and more). The hypothesis about the polygenic type of inheritance of mastitis was confirmed on the basis of histogram analysis of the distribution of the half-sibs and female families on mastitis frequency. The annual realized genetic trend in mastitis varied from 2 up to 9% depending on the cow's age. It is possible and necessary to select dairy cattle to increase resistance to mastitis simultaneously with selection for milk productivity.