Nitric oxide was studied to investigate its possible involvement in the promotion of breast carcinoma: both the development of the primary tumour and the process of metastasis seem to be influenced by the presence and the amount of nitric oxide. We review the available literature on this topic, which seems to suggest an influence of nitric oxide on the cancer cell biology in breast carcinoma, but the argument is still controversial. More studies are needed to clarify the sequence of events and the real impact of nitric oxide on the behaviour of the disease.