Study objective - The aim of the study was to assess and compare the contamination of human breast milk with organochlorine residues through two sentinel practice networks in Lower Saxony, a state of former West Germany, and Saxony-Anhalt, a state of former East Germany. Design - Eligible women were enrolled in this cross sectional study by a network of 51 paedriatric practices in Lower Saxony and 44 in Saxony-Anhalt when bringing their babies for a regular screening examination four to six weeks after delivery. Sociodemographic, lifestyle, and exposure factors were determined by questionnaire. Milk samples were analysed for hexachlorocyclohexane (HCH), hexachlorbenzole (HCB), DDT, dieldrin, polychlorinated biphenyls (PCB), and heptachlorepoxid (HCE); half the samples were also analysed for dioxin. Analytic statistics were computed using multiple logistic regression. Setting - The study was conducted in Lower Saxony, Germany, from July 1992 to June 1993, and in Saxony-Anhalt, Germany, from January to June 1995. Participants - 156 primiparous, breast-feeding women from Lower Saxony and 113 from Saxony-Anhalt were studied, who either were born and raised in former West or East Germany, respectively. Main result - Mean age of mothers and children differed significantly between the two study groups. In Lower Saxony all but two milk samples were well below the tolerable concentrations established by the German Research Council (Deutsche Forschungsgemeinschaft (DFG)). In Saxony-Anhalt no participant had concentrations above those recommended by the DFG. After adjustment for age of mother and child, occupational and non-occuptional pesticide contact, DDT and β-HCH concentrations were significantly lower in Lower Saxony; HCE and dieldrin concentrations were lower in Saxony-Anhalt. No differences between the two states were found for PCB, HCB, γ-HCH, and dioxin. Conclusions - Breast milk contamination levels in former East German Saxony-Anhalt exceeded the contamination in Lower Saxony only for DDT and β-HCH.