For almost 50 years, millions of Mexicans have been directly and/or indirectly exposed to (1,1,1-trichloro-2,2-bis[p-chlorophenyl]ethane) (p,p'DDT). The potential related health outcomes of this exposure are of international concern. The objective of the study was to determine the effect of breast-feeding on serum levels of 1,1-dichloro-2; 2'-bis(p-chlorophenyl)ethylene (p,p'DDE) and p,p'DDT in women of childbearing age who were residents of the state of Morelos, Mexico. In March 1999, we examined a sample of 24 women, ages 21 to 36 years, who came with their children to the outpatient facility of a county Health Center in the state of Morelos. The geometric mean for p,p'DDE was 21.8 ng/ml+/-2.58 (GSD) and 2.9 ng/ml+/-2.84 for p,p'DDT. For each month of breast-feeding we observed similar significant decreases for both p,p'DDE (beta=-0.0403 per log concentration unit, P=0.001) and p,p'DDT (beta=-0.0309, P=0.03) serum levels, adjusted by mothers' age and number of children. The adjusted half-life estimate for p,p'DDE serum levels was 17 months. It was concluded that breast-feeding leads to rapid removal of p,p'DDT from the body (<2 year half-life) compared to nonlactational elimination rates (<5 year half-life). In this population, serum p,p'DDE levels were similar to those found 20 years ago in the United States.