To evaluate in vivo nitric oxide production in Kawasaki disease (KD), urinary nitrite/nitrate (NOx) excretion was measured in 8 children with KD (age 1.1-2.7 years). Urinary NOx excretion was 0.66+/-0.22 mmol mmol(-1) creatinine (mean+/-SD) in the 8 children with KD in the initial stages. The levels were significantly increased compared with those of 12 age-matched healthy control subjects (0.35+/-0.08 mmol mmol(-1) creatinine). Urinary NOx excretion was serially determined in four patients. For each patient, there was a further rise in urinary NOx excretion from baseline levels coincident with the administration of intact-type gammaglobulin and aspirin. With clinical and laboratory improvement, however, urinary NOx excretion declined to the normal range. These findings suggest that endogenous nitric oxide production is enhanced in children with acute KD. Further studies are needed to clarify the role of nitric oxide in the pathogenesis and clinical course of KD.