Despite its unequivocal advantages, breast feeding may be associated with undesired side-effects. Recently, we have shown an association between exposure via mother's milk to dioxins and developmental defects of the child's teeth. The present study was undertaken to analyze further the association between the duration of breast feeding and the occurrence of dental defects. For this purpose, 2 different populations were selected. The first population comprised 40 children who had mineralization defects in the permanent 1st molars, and their age-living area- and sex-matched controls. The median duration of breast feeding was 9 months in the affected children compared to 6 months in the controls. The defects were more extensive after prolonged breast feeding. The second population consisted of 97 children whose mothers had been encouraged to extensive and prolonged breast feeding. Of these children, 24 had mineralization defects. They all had been breastfed longer than 8 months. In both study populations mineralization defects were associated with the duration of breast feeding. The result suggests that long breast feeding may increase the risk of mineralization defects in healthy children, possibly because of environmental contaminants that interfere with tooth development.