Reproduction and health in addition to milk production are the principal factors that are influencing dairy farm profitability. The dairy industry in Iran has changed dramatically in the last decade. The shift toward more productive cows and larger herds is associated with a decrease reproductive efficiency (Heravi Moussavi et al., 2004). Increased knowledge about the principal causes of reduced fertility is essential. It was shown that clinical mastitis during early lactation markedly influenced reproductive performance of Jersey cows (Barker et al. , 1998). Recently, survival analysis was used to assess the effects of diseases on days from calving to conception. Survival analysis allows for a more appropriate management of censored data and time-dependent covariates, which are very common in the study of variables such as days open, days to first breeding, and days from first breeding to conception (Maizon et al., 2004). The objective of this study was to evaluate the effect of postpartum mastitis on days open in Iranian Holstein dairy cows.