OBJECTIVES: To report the incidence of breast abscess in lactating women. DESIGN: Data were combined from two studies (a randomised controlled trial and a survey) to provide a cohort of women for this report. SETTING: Women were recruited from two hospitals on one site in Melbourne, Victoria, the Royal Women's Hospital (public) (1999-2001) and Frances Perry House (private) (2000-2001). PARTICIPANTS: A total of 1193 of 1311 (91%) primiparous, English-speaking women from a diverse range of backgrounds, including those receiving public clinic care, private care and birth centre care. METHODS: A structured telephone interview was conducted on breastfeeding at six months postpartum. MAIN OUTCOME MEASURES: Lactational mastitis and breast abscess. RESULTS: Two hundred and seven women experienced mastitis. Five women developed a breast abscess: 0.4% of women who commenced breastfeeding (95% CI 0.14-0.98); 2.9% of women who took antibiotics for mastitis (95% CI 1.0-6.7). CONCLUSION: Although many authors estimate that 11% of women with mastitis develop a breast abscess, the incidence of lactating breast abscesses in Australia appears to be lower than reported in the past. Our estimate is that 3% of women with mastitis will develop a breast abscess.