The concentrations of polybromodiphenyl ethers (PBDEs) in children at birth (cord blood sera, n = 92) and at the age of 4 years (sera, n = 244) from a cohort established in Menorca Island (Balearic Island, Spain) were studied. This cohort is representative of a general European population that is fed a typical Mediterranean diet. Among the 13 congeners analyzed, BDE #47 was the most abundant in both types of samples, with mean values of 2.8 ng/g of lipid weight in cord blood sera and 2.9 ng/g of lipid weight in sera. The observed distributions of PBDEs paralleled the composition of the commercially available mixtures of pentabromodiphenyl ethers. The concentrations of most congeners were higher in females than in males, but the differences were not significant. PBDE in the sera of 4 year old children was higher among those having been fed with maternal milk than formula. The differences were statistically significant for the congeners found in higher concentrations (e.g., BDE #47 and BDE #99). This difference was consistentwith previous reports on polychlorobiphenyls or 4,4'-DDE, indicating that despite the short lactation period (about 4.5 months as an average in this cohort), breastfeeding was the determining factor for the body burden of these compounds at 4 years of age. The observed increases of average body burden of total PBDEs between birth and the first 4 years of growth were 65 and 10 ng for breastfed and formula fed children, respectively.