The Wnt genes encode a family of related, secreted proteins which initiate a signal cascade upon binding to cell surface receptor molecules. The signaling pathway has been shown to be critical for normal growth and development in model organisms and is implicated in the genesis of numerous human cancers. Wnt proteins regulate mammary development in the mouse but their precise role in normal breast development and malignant transformation in humans remains poorly defined. In this study, we have examined the expression of several Wnt ligands by in situ anti-sense RNA hybridization in normal and malignant human breast tissue, as well as in several estrogen-responsive and estrogen-independent human breast cancer cell lines. The specific Wnt genes tested included Wnt1, Wnt2, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7b and Wnt10b. We have also studied the expression of frizzled receptors 1 and 2 by immunohistochemistry in these tissues. Our results indicate that several of the Wnt ligands, especially Wnt1 and Wnt6, are strongly expressed in both normal and malignant breast tissue and that Wnt7b is down-regulated in breast cancer, compared to normal breast epithelium. The expression of frizzled 1 and 2 receptors was found to be up-regulated in breast cancer. These studies provide additional support to the hypothesis that the Wnt signaling pathway is involved in human breast cancer.