Genetic parameters for number of clinical mastitis (CM) cases per cow adjusted for a standardized lactation length of 305 days were estimated from data of 6242 Holstein cows with 9550 lactations. Single-trait and three-trait animal models were used including parity, herd × two-year period of calving and calving season as fixed effects and the additive genetic and permanent environmental effects of the cow and the residual effect as random effects. A heritability of 0.11 was estimated from both models and the genetic correlations of this trait with 305-d milk yield and lactation mean somatic cell score were 0.34 and 0.80, respectively.