We report the occurrence as well as biosynthesis of a pituitary hormone, follicle stimulating hormone (FSH) in human breast. Using immunoperoxidase localization technique, both FSH and beta-FSH were localized in cytoplasm of epithelial cells but not in stromal cells. Immunostaining was more intense in benign and malignant specimens as compared to normal. In vitro radiolabelled precursor experiments with breast tissue explants indicate de novo synthesis of FSH. Human milk had higher concentrations of FSH as compared to serum. In gonads, FSH is involved in the cellular growth, differentiation and function. The presence of higher levels of FSH in benign mammary tumors and breast cancer when compared to normal breast supports the suggestion that FSH might have a role in the process of breast malignant transformation.