The objective of this study was to present a relationship between mastitis in the post-calving period and fertility traits of dairy cows. The threshold of 200,000 cells/ml for somatic cell count is used as a reference for healthy quarters of the cow’s udder. The genetic correlation between mastitis and somatic cell count is strong (from 0.7 to 0.8). Although heritability for fertility traits is low (from 0.01 to 0.02), the genetic standard deviation for mastitis varies from 1.2 to 7.0 percentage units, suggesting that genetic gain can be achieved by selecting for mastitis. Results of this study suggest that mastitis in the postpartum period can have a markedly negative impact on the reproductive performance of dairy cows. The synergistic effect of common conditions (somatic cell count, body condition score and lameness score) or other factors (e.g. heat stress, fertility management, the presence of repeat breeders) also lowers fertility of cows. Production of milk with fewer than 100,000 cells/ml leads to improved health and fertility in the cows.