We investigated whether early childhood factors such as breast-feeding, parity, and smoking contribute to the variation of organochlorine compounds (OC: dichlorodiphenyldichloroethene, hexachlorobenzene, beta-hexachlorocyclohexane, and the sum of polychlorinated biphenyls including the congeners 101, 118, 138, 153, 170, 180, 183, and 187) at approximately 7 y of age. OC were measured in whole blood of 350 children. Pregnancy characteristics and the child's living conditions were gathered by questionnaires administered to the parents and interviews with the mother. Height and weight were determined during the medical examination. Exclusion of incomplete data and nonbiologic children of the mothers yielded a sample of 337 children. We applied regression analysis with indicator variables, controlling for confounders. No systematic association was detected for birth order or maternal smoking during pregnancy. The OC concentrations are diluted in children with a higher body mass index (>18 kg/m(2)). We found a strong, dose-dependent relationship between the duration of breast-feeding (none, 1-4 wk, 5-8 wk, 9-12 wk, >12 wk) and the concentration of all five OCs. Of the potential determinants analyzed, more of the variance of the OC concentration is accounted for by breast-feeding than by any other variable. Exclusive breast-feeding beyond 12 wk was associated with a doubling of OC whole blood concentration compared with bottle-fed children (dichlorodiphenyldichloroethene, 0.28 microg/L versus 0.55 microg/L; beta-hexachlorocyclohexane, 0.05 microg/L versus 0.14 microg/L; hexachlorobenzene, 0.14 microg/L versus 0.25 microg/L; sum of polychlorinated biphenyls, 0.25 microg/L versus 0.55 microg/L). These findings indicate that breast-feeding can lead to an extended duration of increased OC exposure in childhood.