Breast cancer is a serious illness affecting approximately one in nine women in the United States. Although an actual cause for breast cancer is unknown, genetic and environmental factors have been associated with its onset. Elevated levels of estrogen and heightened expression of the WNT10B proto-oncogene have been implicated in the development of human malignant breast tumors because they enhance the proliferation of mammary tissue. Two pyrethroid insecticides, sumithrin and fenvalerate, have been shown to mimic estrogenic activity in MCF-7 human breast carcinoma cells by inducing pS2 expression whereas two other pyrethroids, permethrin and d-trans allethrin do not have the same capability. To investigate if estrogen and these four pyrethroid insecticides could affect the expression of a gene related to mammary gland development, WNT10B expression in pyrethroid-treated MCF-7 cells was examined. MCF-7 cells under normal growth conditions do not express WNT10B. Reverse-transcriptase polymerase chain reaction (RT-PCR), nested PCR and Southern hybridization were employed to detect WNT10B expression. As controls, cells were treated with either ethanol, corn oil, or Vista LPA solvent. When compared to the solvent-treated controls, sumithrin, fenvalerate and estrogen treated MCF-7 cells all had increased levels of WNT10B expression. The non-estrogenic pyrethroids, d-trans allethrin and permethrin, demonstrated a similar elevation of WNT10B expression at a lower concentration, but not at the higher concentration. The results suggest that pyrethroid insecticides and estrogen can enhance the expression of the WNT10B proto-oncogene. However, since both the estrogenic and non-estrogenic substances amplified Wnt10B expression, the mechanism likely involves multiple distinct pathways.