Allergic disease (AD), including atopic eczema, asthma, allergic rhinitis, and food allergy, is characterized by an imbalance between cytokines produced by distinct T-helper cell subtypes. Whether this imbalance can be transferred from mother to breast milk remains to be established. The objective was to investigate the concentrations and interactions of nutritional and inflammatory factors in breast milk. Breast milk samples were collected from mothers with AD (n= 43) and without AD (n= 51). The concentrations of transforming growth factor (TGF)-&bgr;2, tumor necrosis factor-&agr;, IL-4, IL-10, prostaglandin E2, and cysteinyl leukotrienes were measured by immunoassays and fatty acid composition by gas chromatography. Mothers with AD had a lower concentration of TGF-&bgr;2in breast milk [median (interquartile range), 420 (278–701) ng/L] compared with those without AD [539 (378–1108) ng/L;p= 0.003], whereas other cytokines, prostaglandin E2, and cysteinyl leukotriene concentrations or fatty acid composition were not significantly different between the groups. The breast milk inflammatory factors and fatty acid composition were shown to be related. A positive association was observed between TGF-&bgr;2and the proportion of polyunsaturated fatty acids (p= 0.038) and a negative association between TGF-&bgr;2and the proportion of saturated fatty acids (p= 0.029) in breast milk. The reduced TGF-&bgr;2concentration in the breast milk of mothers with AD may interfere with the development of the mucosal immune system of the breast-fed infant. The observed associations between nutritional and inflammatory factors in breast milk suggest that it may be possible to influence the immunologic properties of breast milk by dietary intervention of the mother.