This study aimed to highlight milk production loss caused by mastitis in dairy farms of Blida region, Algeria. The estimation of losses was made using the coefficient of loss calculated according to the model reported by Seegers et al. (1999) and Mtaallah (2000) which took into account the cell counts of the farm milk and its milk production measured on the day of the survey. Milk production loss of dairy farms assessed on the basis of a regional milk production of 38723 x 103 liters (L) for the year 2008 and a loss coefficient of 0.23 with a confidence interval of 0.19 ≤ µ ≥ 0.27, would be included in the interval 7357370 and 10455210 L. It is equivalent to a financial loss between 323724280 and 460029240 Algerian Dinar (AD) or 4.05 and 5.7 million US dollars (at a rate of 1 US $ = 80 AD). The loss in National milk production, assessed on the basis of a national milk production of 2.207 x 109 L for the year 2008 and a loss coefficient of 0.23 with a confidence interval of 0.19 ≤ µ ≤ 0.27, would be between 419.33 x 106 and 595.89 x 106. It is equivalent to a financial loss between 18450.52 and 26219.16 million AD or 230.6 and 327.7 million US dollars. At the end, the implementation of preventive and curative strategies in addition to further complementary investigations will be of great importance in controlling mastitis and in limiting the losses.