Sodium/potassium (Na/K) ratios are considered to be a marker of mammary epithelial paracellular permeability. The aim of the present study was to investigate the association between maternal atopy and Na/K ratios in breast milk and the association between Na/K ratios in breast milk and the development of atopy in the offspring. Early and mature milk samples were obtained from 30 atopic and 43 non-atopic women. We found no differences in the Na/K ratios between atopic and non-atopic women. At 18 months of age, 22 (30%) of the children had a positive skin prick test (SPT) and 26 (36%) had symptoms of atopic diseases. Overall, high levels of Na/K compared with low and slightly raised levels of Na/K in the maternal milk tended to be associated with a positive SPT and atopic disease. However, if the mother was atopic, high levels of Na/K in early or mature milk were associated with a significantly increased risk of a positive SPT or atopic disease in the offspring [RR = 4.8 (1.9-12)] whereas no such association was observed in non-atopic mothers [RR = 0.8 (0.4-1.7), p for interaction = 0.001]. Thus, high Na/K levels in the breast milk may be associated with the development of atopy and atopic diseases in the offspring of atopic mothers.