Clinical mastitis is a serious disease of the udder which influences dairy cattle, especially in the early section of lactation. Cattle breeders spend annually around 1 500 000 000 Czech crowns in relation to clinical mastitis. Mastitis is divided into clinical and subclinical forms. Clinical mastitis has local symptoms (redness, heat and swelling) and system symptoms (fever, anorexia and exhaustion). Dairy cattle can die after infection due to sepsis. Subclinical mastitis has a one symptom increasing count of somatic cells in milk. The aim of the study was to evaluate the phenotypic effect of linear type traits on somatic cell count and the incidence of clinical mastitis in Holstein cattle. Data about the incidence of clinical mastitis was collected on 6 farms from 1997 to 2020. Clinical mastitis was monitored in the individual sections of the first lactation and in the sum of lactation. We used for our analysis data from milk recording (somatic cell count). The evaluation of cattle was based on a methodology published by the Holstein Cattle Breeders Association of the Czech Republic. In accordance with available literature and our results, we suggested an appropriate exterior of Holstein cattle. The biggest effect of mastitis has front udder attachment and udder depth. The incidence of mastitis significantly increased with increasing depth of udder. Clinical mastitis can be included in the breeder program as a new trait in the Czech Republic. It precedes this step national monitoring of clinical mastitis on farms. In this case, can be used data from the web application “Deník nemocí a léčení” provided by the Czech-Moravian Breeders’ Corporation.