OBJECTIVES: To estimate the incidence of mastitis in breastfeeding women during the first six months after delivery, and to identify the healthcare services used and treatments received by women with mastitis. DESIGN: A prospective cohort study with questionnaire and telephone follow-up. SETTING: A teaching hospital and the only private hospital offering obstetrics at the time of the study in the Lower Hunter Region, New South Wales. PARTICIPANTS: 1075 women in postnatal wards following delivery of a single infant who were breastfeeding at the time of recruitment. RESULTS: 233 women returned a questionnaire indicating they believed that they had developed mastitis during follow-up. Of these, 219 had mastitis by the study criteria, giving an estimated crude incidence of 20% (95% CI, 18%-22%) in the six months after delivery. Most cases of mastitis (75%) occurred within seven weeks after delivery. Multiple episodes of mastitis were reported by 63 women: 27 reported three or more episodes during six months. 160 women (73%) reported consulting a general practitioner about their mastitis. Despite National Health and Medical Research Council guidelines that antibiotics be continued for 10 days, 148 (87%) of the 170 women who were prescribed antibiotics for the first episode of mastitis took them for less than 10 days. CONCLUSIONS: A large proportion (20%) of women who were breastfeeding developed mastitis in the six months after delivery. As general practitioners are commonly consulted about mastitis, it is important that they know how to diagnose and treat this condition.