Background: Breast-feeding may affect the risk of developing allergy during childhood and may also cause exposure to immunotoxicants, such as polychlorinated biphenyls (PCBs), which are of concern as marine pollutants in the Faroe Islands and the Arctic region. Objectives: The objective was to assess whether sensitization and development of allergic disease was associated with duration of breast-feeding and prenatal or postnatal exposures to PCBs and methylmercury. Methods: A cohort of 656 singleton births was formed in the Faroe Islands during 1999-2001. Duration of breast-feeding and history of asthma and atopic dermatitis were recorded at clinical examinations at ages 5 and 7 years. PCB and mercury concentrations were determined in blood samples obtained at parturition and at follow-up. Serum from 464 children (71%) at age 7 years was analyzed for total IgE and grass-specific IgE. Results: The total IgE concentration in serum at age 7 years was positively associated both with the concomitant serum PCB concentration and with the duration of breastfeeding. However, the effect only of the latter was substantially attenuated in a multivariate analysis. A raised grass-specific IgE concentration compatible with sensitization was positively associated with the duration of breast-feeding and inversely associated with prenatal methylmercury exposure. However, a history of asthma or atopic dermatitis was not associated with the duration of breast-feeding, but children with atopic dermatitis had lower prenatal PCB exposures than non-allergic children. Conclusions: These findings suggest that developmental exposure to immunotoxicants may both increase and decrease the risk of allergic disease, and that associations between breast-feeding and subsequent allergic disease in children may, at least in part, reflect lactational exposure to immunotoxic food contaminants.