over a period of one year (Nov 2007. Oct,2008) a total of 1757 Holstein -Friesien cows belonging to a commercial dairy farm in Giza -Egypt were included in this investigation, Somatic cell count (SCC) as a measure of subclinical mastitis was recorded for three times in three consecutive months, Bacteriological examination was also done for milk sample. Reproductive data including days to first insemination, number of inseminations per conception (S/C) and days open (DO) were individually recorded also the percentage of cows conceived by 305 days in milking (DIM) and percentage of culled cows for reproductive failure were recorded. The obtained data were recorded. The animal status (parity and reproductive status) as well as the establishment and degrees of affection were also considered. The obtained results were recorded and statistically analyzed. Results revealed that, the incidence of subclinical mastitis was 31.75 and 23.79 % of the total lactating non pregnant and lactating pregnant cows, respectively, Among cases of subclinical mastitis, 75 % were bacteriolo-gically positive. Coagulase-negative staphylococci, Staphylococcus aureus, Streptococcus uberis and streptococcus dysgalactiae were the prevalent isolates. The adverse effects of subclinical mastitis on the reproductive performances in dairy herd included prolonged days ta first insemination; Days open; increased number of inseminations per conception and culling rate for infertility. The establishment of subclinical mastitis during the interval to first insemination prolonged this Stage while, the establishment from the first insemination to the fertile one prolonged the days open and increase the number of inseminations per conception. It could be concluded that, subclinical mastitis was associated with unfavourable effects on reproductive performance of dairy cows. Therefore, the appropriate management of lactating dairy cows to minimize the incidence of mastitis should increase the profitability of dairy herds not only by improving milk quality, reducing the use of antibiotics, reducing the amount of milk discarded, and reducing involuntary culling, but also by improving reproductive performance.