Melatonin is a pineal hormone that strongly inhibits the growth of breast cancer cells in vitro and in vivo. We report thefirst use of immunohistochemical analysis to determine the distribution of the high-affinity melatonin receptor subtype, MTI, in human breast tissue, the hypothalamic suprachiasmatic nucleus, and skin. The MT1 antibody, which is specific for the cytoplasmic portion of the receptor, produced cytoplasmic staining in normal-appearing breast epithelial cells and ductal carcinoma cells; stromal cells, myoepithelial cells, and adipocytes were nonreactive. The majority of nonneoplastic samples (13/19 [68%]) were negative to weakly positive, while moderate to strong reactivity was seen in most cancer samples (49/65 [75%]). Thus, although MT1 receptors were detectable in normal and malignant breast epithelium, high receptor levels occurred more frequently in tumor cells (P < .001), and tumors with moderate or strong reactivity were more likely to be high nuclear grade (P < .045). These findings may have implications for the use of melatonin in breast cancer therapy.