BACKGROUND: Mastitis is an important health disorder in dairy herds which affects milk production, animal reproduction and production. OBJECTIVES: The aim of this study was to estimate the cumulative incidence of mastitis in dairy herds in Tehran province.  METHODS: From 679 dairy herds in Tehran province that have more than 50 animals, 32 dairy herds were selected randomly and animals that had calved from March 2007 to March 2008 were followed until the next calving or culling. In total 10506 animals were included in the study. The information on number, parity, occurrence of mastitis was gathered from registry forms. RESULTS: Cumulative incidence of mastitis in this study was calculated 35.8% (95% Confidence interval (34.9-36.8), 3764 animals of 10506). Incidence of mastitis was increased by the increasing parity of animals, and animals in fifth parity have the most incidence. CONCLUSIONS: The results of this study showed the incidence of mastitis in this area is very high and it is necessary to control it.