The concentration of nitrate (the stable oxidation product of nitric oxide) in plasma and its excretion in urine was measured in 20 patients with a diagnosis of gastroenteritis. On day 1 of the illness plasma nitrate concentration was significantly elevated compared with a healthy control population (92.7 +/- 17.0 mumol/l vs. 33.1 +/- 1.6 mumol/l; P < 0.001) and continued to be elevated on days 2 and 3. Urinary nitrate excretion was also elevated. The plasma nitrate concentration correlated with disease severity as assessed by stool frequency and plasma urea concentration. Plasma nitrate concentration may be a sensitive and clinically useful indicator of severity of gastroenteritis.