The aim of this study was to examine the exposure of lactating mothers to aflatoxins using aflatoxin M(1) in breast milk as a biomarker for exposure and to detect its determining factors and possible effects on child growth. A 9% sample of 2022 lactating women who exclusively breastfed their infants, including 91 lactating women of urban areas of Tabriz city and 91 lactating women of its rural areas were recruited by a geographical clustered sampling method. Breast milk samples and information on food intake were collected from subjects using structured food-frequency questionnaire. Extraction of AFM(1) was performed with the ELISA test procedure. Aflatoxin M(1) was detected in breast milk of 20 out of 91 mothers (22%) in concentrations of 6.96 +/- 0.94 (pg/ml) in rural areas. Aflatoxin M(1) contamination was not present in samples of urban areas. The presence of aflatoxin M(1) was significantly associated with consumption of local milk (beta = 0.71, P < 0.001) and stunted growth in children (beta = -0.31, P < 0.015). These findings emphasize the need for developing strategies to reduce exposure to aflatoxin, possibly involving interventions targeted at reducing contamination of foods.