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The aim of the study was to investigate factors influencing days to first insemination and days open (DO) with a special focus on the interaction between mastitis and reproductive performance. Throughout a period of 2 years, disease occurrence and reproductive parameters in 15 commercial dairy farms were recorded. Sterile quarter milk samples were collected monthly for cytobacteriological examination. The data set included the first two samplings after parturition, clinical mastitis occurring during the postpartum period and reproductive records. To avoid bias, cows with less than 42 days to first insemination (DFS) were excluded from analysis. Data of 1362 lactations were analysed by using a linear mixed model. During the postpartum period, 11.0% of cows developed clinical mastitis, and 33.6% subclinical mastitis in the period up to 42 days in milk (DIM). Mean calving to first insemination interval was 81.1±31.3 days and was significantly influenced by herd x season (P<0.01), parity (P<0.01), milk yield (P<0.05), reproductive (P<0.001) and metabolic disorders (P<0.001). In cows with subclinical mastitis the interval was prolonged by 11.7 days (P<0.01), whereas clinical mastitis had no effect. Mean days open were 104.6±51.3 days. Factors with significant influence on days open were herd×season, milk yield, as well as reproductive and metabolic disorders. Clinical and subclinical mastitis did not increase days open. Subclinical mastitis during early lactation prolonged the interval to first insemination, whereas clinical mastitis had no significant effect on reproductive performance.