Hypothesis. The increased excretion of lactose in urine will be an accurate predictor of increased breast permeability during inflammation of the breast and therefore could predict whether there is a relationship between the severity and type of symptoms suffered during mastitis and changes in breast permeability. Methods. Twenty-six mothers were followed prospectively from Day 5 postpartum to the end of their lactation. Milk from each breast, blood, 24-hour urine samples, and data on breast and systemic pathologies were collected at reference intervals during the first three months postpartum, daily during the occurrence of any breast inflammation and 7 days after resolution of symptoms. Results. The 24-h excretion of lactose in urine was higher during mastitis ( p< 0.001), peaking at the commencement of the mastitis and decreasing over time until there was no significant difference at the time of follow-up when compared to mothers without mastitis ( p< 0.25). The changes in lactose in urine were consistent with the changes in sodium, chloride, and lactose in milk, confirming milk expressed for sample analysis was representative of milk from the site of inflammation. Increased severity of breast symptoms experienced during mastitis was a significant predictor for an increased excretion of lactose in urine ( p< 0.022). Conclusions. The use of 24-h excretion of lactose in urine provided a reliable marker of changes occurring in breast permeability and subsequently has shown that an increasing area of inflammation of the breast was predictive of an increase in breast permeability and associated changes in milk composition.