Background:Limited information is available about the frequency and risk factors for lactation mastitis. The purpose of this study was to investigate frequency, risk factors, and outcome for lactation mastitis.Methods:We analyzed data from a cohort study of 350 mothers with healthy term infants, who were followed up for one year postpartum.Results:Eighty-three women (23.7% of the sample) reported one or more episodes of mastitis symptoms, and 61 (17.4%) women reported symptoms including fever. Fifty-six women (16%) received one or more courses of antibiotics for mastitis. Thirty women (8.5%) of the total cohort had recurrent episodes of mastitis symptoms. A total of 123 episodes of mastitis symptoms were reported. No cases of abscess were reported. Using multiple logistic regression, the adjusted risk ratio for mastitis symptoms associated with sore nipples in the first month was 2.07 (95% CI = 1.17, 3.66). A reduced risk of mastitis was associated with maternal smoking during pregnancy, with supplementation with water in the first month, and with the use of a pacifier on a daily basis within the first month. A history of mastitis symptoms was associated with a longer overall duration of breastfeeding.Conclusions:Our results suggest that mastitis may be a marker for an ample milk supply. Mothers presenting with mastitis can be reassured that the outlook for continued successful lactation is good. Further research is needed to investigate the reasons for recurrence of mastitis and the etiology of episodes occurring late in lactation.