In this study we investigated the immunohistochemical expression of eNOS and nNOS in 80 invasive breast carcinomas. Since NO is known to influence apoptosis and angiogenesis we also determined the apoptotic index of the tumor cells by the TUNEL method and tumor angiogenesis by immunostaining the sections with FVIII-related antigen and assessing the number of positively stained vessels. Respectively, 65% and 11% of the cases expressed cytoplasmic positivity for eNOS and nNOS. The mean apoptotic index of the tumors was 0.60%. eNOS or nNOS expression did not associate with the apoptotic index (p=0.36 and p=0.58, respectively). Cases with a positive estrogen or progesterone receptor status were significantly more often eNOS positive than receptor negative cases (p=0.012 and p=0.015, respectively). No association was found between the estrogen or progesterone receptor status and nNOS expression. Neither eNOS nor nNOS expression was associated with vascular density, tumor grade or the TNM status of the tumors. The results show that eNOS and nNOS are expressed in breast carcinomas possibly contributing to their NO synthesis. No association with apoptosis or angiogenesis was detected. eNOS expression was, however, associated with the positive estrogen and progesterone receptors status suggesting that its synthesis might be regulated by hormonal stimulation.