Nitric oxide synthases are expressed in breast cancer. To elucidate the clinical role of the inducible NOS (i-NOS) in human breast cancer, 161 primary breast cancer tissues were stained immunohistochemically. Staining patterns for i-NOS were correlated with classical prognostic factors such as lymph node status, age, hormonal receptor status, tumour size and tumour differentiation. With classical prognostic factors such as lymph node status, age, hormonal receptor status, tumour size and tumour differentiation. Patients survival was also analysed. Sixty-one percent of the tumours stained positively for i-NOS. Detection of i-NOS was positively correlated with increasing tumour size and decreasing tumour differentiation (P=0.018 and P=0.039, respectively). However, in the ⩽50 year age group, i-NOS staining also correlated with lymph node status. Patients with i-NOS-positive breast carcinomas had a significantly worse overall survival rate versus those with negative stains (5-year survival rate 84.8% versus 67.1%; P=0.049; log-rank test). To date, this is the largest analysis of i-NOS expression in breast cancer patients and the only study to assess survival.