The aim of the present study was to study the effect of the season, period and number of lactation on mastitis incidence and milk yields in dairy cows. The climate in Bulgaria implies the presence of seasonal variations in mastitis prevalence in dairy cows, reared in free-stall barns all the year round: spring – 5.33%, summer – 4.9%, autumn – 4.2% and winter – 4.5%. The mastitis incidence during the lactation periods increased gradually from calving to the 120th lactation day. For the first 120 days of lactation, the mastitis incidence attained 26%. The reduction in milk yields after the lactation peak was associated with gradual decliine in cows affected by mastitis to 1.7% during the last month of lactation. With increasing the number of lactations, mastitis incidence also increased: 23.3% in 1st lactation cows, 26.7% in 2nd lactation cows, 48.9% in 3rd lactation cows and 43.3% in cows in fourth or subsequent lactation. The highest daily milk yield at the peak of lactation (34.05 kg) was established in cows affected by mastitis after the 180th lactation day, followed by cows with mastitis between the 60th and 180th lactation days (32.54 kg), and third came healthy cows (31.98 kg). The lowest daily milk yield at the peak of lactation was demonstrated in cows affected by mastitis in the first 60 days of the lactation (29.39 kg). It was proved that cows with exceptionally high milk yields in the beginning of lactation were more prone to mastitis at a later stage due to weakened systemic resistance and consequent disease. Cows with relatively lower milk yield during the lactation period (7580.3 kg) were more resistant to environmental factors, therefore were not affected by mastitis and produced highquality milk with lower costs vs those with highest milk yields affected by mastitis after the 180th day. These cows produced more milk (7607.9 kg) within a 305- day lactation, but the treatment costs could be hardly compensated. Cows in which mastitis occurred until the 60th and between the 60th and 180th lactation day, produced 7215.5 and 7484.1 kg milk, respectively. The average milk yield per lactation of healthy cows was by 137.83 kg higher than that of cows with mastitis. A statistically significant reduction in milk fat (Р < 0.05) and milk protein (Р < 0.001) between healthy and diseased cows was observed.