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Abstract. Title of the paper: Relationship between milk yield, fertility and amount of disease of Holstein Friesian dairy cows Data of 1 841 lactations of 353 Holstein Friesian dairy cows and data according to their reproductive fertility and diseases were analysed over a period of five years in a breeding farm in Thüringen (Germany). As a result the lowest success of first service has been achieved at a lactation (ranged) between 9 000 and 11 000 kg. The time between calving and the following first service varied according to the first disease in last lactation. With rising milk yield the frequency of diseases and the success of first service tend to decrease. Cows achieving a 305 day milk yield of more than 11 000 kg showed with 56 % the lowest frequency of diseases. In average 60 % of all dairy cows need to be treated once in a lactation. The diseases of highest prevalence are in the field of reproductive fertility and udder health. For heifers a decreased success of first service has been detected if they showed an increased prevalence of illness.