The role of calcium independent inducible nitric oxide synthase (iNOS) in breast carcinoma is controversial, and the implications of iNOS expression on prognosis are not known. In this study, we aimed to investigate the significance of immunohistochemical iNOS expression in 100 invasive ductal carcinomas. In addition, 11 normal breast tissues, 20 cases of usual ductal hyperplasias (UDHs) and 20 fibroadenomas were included. We found that 78% of malignant and 75% of benign cases showed iNOS immunoreactivity. However, the intensity and the quantity of iNOS expression were significantly higher in the cancer group when compared with benign breasts (P<0.001), suggesting a role of iNOS in breast carcinogenesis. We were unable to show a correlation between iNOS expression and tumor grade, axillary lymph node status, and estrogen receptor expression. In 50 axilla negative cases having 5--12 years follow-up, disease free survival (DFS) rate was significantly lower in cases showing strong iNOS expression (P=0.05). As strong iNOS expression was correlated with short DFS, we concluded that further studies would be necessary to elucidate if iNOS expression might be a useful prognostic marker in breast carcinoma, especially in the axilla negative group.