The objective of this study was to determine the effects of clinical mastitis on REPRODUCTIVE PERFORMANCE in Holstein dairy cows in IRAN. Therefore we collected data from one large dairy farm (4200 dairy cows) from March 2008 until June 2010.The animals were divided into two groups according to ccurrence of mastitis as follows: clinical mastitis cows which occurred before pregnancy as group I (n=1248), cows without any mastitis selected as control, group II (n=1347), then collected information were analyzed using SPSS.various REPRODUCTIVE indices such as the days of calving to first services (DFS), calving to conception interval (CCI) services per conception (S/C), pregnancy percent in various intervals (120 & 150 days) after calving (PP<120, PP<150) and mastitis prevalence in various seasons and months of the year were analyzed and compared with control group. Also the pregnancy rate of services almost two months before and after mastitis was studied. Then we used T test, chi square and Anova test to analyze two groups. Based on our results, in this study the great prevalence of mastitis was in summer and the least in winter (P£0.001). And also the great prevalence of mastitis was in September (Shahrivar) and the least was in march (Esfand) (P£0.001). Results indicated the undesirable REPRODUCTIVE indices in the mastitis cows compared with the control group.Statistical analysis of results showed that there is a significant difference between two groups (mastitis vs control) (P£0.001) in the most instances, such as DFS, CCI, S/C, PP<120, PP<150, as DFS: 63 days vs.43 days, CCI: 203 days vs.82 days, S/C: 3.6 vs.1.8, PP<120: 31% vs.80%, PP<150: 45% vs.88% respectively.