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The aim of the present study was to investigate the effect of subclinical mastitis (somatic cell count (SCC)>250000 cells/ml) on the fertility of Holstein dairy cows in different parities, lactation stages and seasons. Data of 2437 SCC in a dairy farm including number of insemination, pregnancy detection, date of calving and insemination, SCC after insemination (maximum of 30 days), and parity were evaluated. The rate of subclinical mastitis in the first, second, third and fourth and more lactation number was 18.8%, 25.7%, 26.5% and 35.4%, respectively and in spring, summer, autumn and winter was 24.5%, 19.7%, 18.5% and 33.6%, respectively. Conception rate of cows with subclinical mastitis (24.6%) was lower than cows without subclinical mastitis (30.9%; P=0.003). In addition, the conception rate in the second and third lactation in cows with subclinical mastitis was significantly lower than in cows without subclinical mastitis. The conception rate of cows with subclinical mastitis in summer in the second lactation and in winter in the second and third lactation was significantly lower than in cows without subclinical mastitis. Therefore, subclinical mastitis in mid lactation stage and also in the second and third parities especially in summer and winter can disturb fertility more than the other conditions. Also, considering these facts during these periods of times in managing of subclinical mastitis can improve fertility outcome of the herd.