Expression of inducible nitric oxide synthase (iNOS) by tumor cells has been suggested to abrogate metastasis in several tumor models, whereas constitutive NOS expression correlated positively with tumor grade in human breast carcinoma. Whether or not expression of one of the various NOS isoforms could predict the prognosis of breast cancer, however, has not been established. In the present report we investigated the cellular distribution of NOS isoforms in a series of benign and malignant breast tumors and in normal breast tissue. Immunohistochemistry revealed that in samples of benign disease the number of iNOS+ epithelial cells or total epithelial cells was 69+/-16% (n = 50). In samples of grade II invasive ductal breast carcinomas the number of iNOS+ tumor cells or total tumor cells was 62+/-20% (n = 40), compared to 12+/-9% (n = 40) in samples of grade III carcinomas (P<0.0001). iNOS protein was also identifiable in most of the epithelial cells of normal breast tissue (n = 4). In contrast, eNOS protein was restricted to vascular endothelial cells in all of the specimens studied. Since the presence of tumor cell iNOS protein is inversely related to the tumor's metastatic potential, we conclude that endogenous tumor cell mediated iNOS expression might have an inhibitory effect on the metastatic process in breast cancer.