Mastitis, an inflammation in the breast, has recently been linked with higher human immunodeficiency virus (HIV) load in breast milk and higher risk of mother-to-child transmission of HIV. Among 334 HIV-infected women in Malawi who were breastfeeding, the prevalence of mastitis, as indicated by elevated breast milk sodium, was 16.4% at six weeks and 2.8% at six months postpartum. Mastitis is associated with significantly higher concentrations of immunological and inflammatory mediators in breast milk, including lactoferrin, lysozyme, secretory leukocyte protease inhibitor, interleukin-8, and RANTES. Mastitis is potentially preventable by improving micronutrient status of breastfeeding women and can be treated with antibiotics and clinical management. These studies in Malawi suggest that mastitis may contribute to transmission of HIV through breast milk.