Urinary nitrate is known to be a measure of environmental exposure. Only recently an endogenous metabolic pathway resulting in nitrate synthesis was described. We have determined the concentration of nitrate expressed as nitrate/creatinine ratio in the urine of patients in whom a significant environmental nitrate load was presumed to be absent. A significant elevation of nitrates in patients with inflammatory bowel disease compared with patients with non-inflammatory disorders was observed (0.173 +/- 0.155 vs. 0.037 +/- 0.016 mol/mol creatinine, P < 0.01). Increased nitrate excretion in the urine of patients with inflammatory bowel disease may reflect the endogenous nitrate synthesis induced by the immune activation.