Previously, we have found that subclinical breast inflammation, as indicated by raised breastmilk concentrations of sodium and the inflammatory cytokine, interleukin-8 (IL-8), was highly prevalent in Bangladesh and associated with poor infant growth. In order to investigate further the prevalence of subclinical breast inflammation and to assess the impact of dietary intervention, we studied rural Tanzanian women taking part in a study of dietary sunflower or red palm oil supplementation during late pregnancy and lactation. We measured breastmilk concentrations of IL-8, the anti-inflammatory cytokine, transforming growth factor-β2 (TGF-β) and the ratio of sodium to potassium. We also estimated systemic inflammation by plasma concentrations of the acute phase proteins, α1-acid glycoprotein and C-reactive protein. There were highly significant intercorrelations among milk Na/K ratio and concentrations of IL-8 and TGF-β, the last only after treatment with bile salts which also improved TGF-β recovery in the enzyme-linked immunosorbent assay (ELISA). Plasma acute phase protein concentrations tended to correlate with milk Na/K ratio and IL-8, suggesting that subclinical breast inflammation was related to systemic inflammation. Dietary supplementation with vitamin E-rich sunflower oil but not provitamin A-containing red palm oil decreased milk Na/K, IL-8 and TGF-β at 3 months postpartum; however, the effect was significant only for Na/K ratio. The results suggest that milk Na/K ratio, IL-8, and TGF-β all measure the same phenomenon of subclinical breast inflammation but that Na/K ratio, having the lowest assay variability, is the most useful. Subclinical breast inflammation may result in part from systemic inflammation and may be improved by increased dietary intake of vitamin E-rich sunflower oil.