Recently, we have shown that low adiponectin levels are significantly associated with an increased breast cancer risk. It seems to be very important to study the expression of adiponectin receptor 1 (AdipoR1) and receptor 2 (AdipoR2) in the human breast epithelial cells and breast cancer cells in order to clarify whether or not adiponectin exerts its effects directly on these cells. Expression of adiponectin, AdipoR1, and AdipoR2 mRNA was determined by RT-PCR assay using the RNA samples obtained from human breast cancer cell lines (MCF-7, T47D, SKBR3, and MDA-MB231), HMEC (primary culture of normal human mammary epithelial cells), adipose tissues (axilla) as well as breast cancer cells and normal breast epithelial cells selectively collected from breast cancer tissues by laser microdissection (LMD). Adiponectin mRNA expression was observed only in the adipose tissues. On the other hand, AdipoR1 and AdipoR2 mRNA expression was observed in all four breast cancer cell lines, HMEC, adipose tissues as well as breast cancer cells and normal breast epithelial cells selectively collected by LMD. In addition, AdipoR1 and AdipoR2 expression in both normal breast epithelial cells and breast cancer cells was confirmed by immunohistochemistry. These results suggest a possibility that adiponectin might modulate the growth of normal breast epithelial cells and breast cancer cells directly through AdipoR1 and AdipoR2 receptors, and that the association of low serum adiponectin levels with a high breast cancer risk might be explained, at least in part, by the direct effect of adiponectin on the breast epithelial cells.