Background: Recent studies have documented increased release of endothelin(ET) during acute attack of asthma. The purpose of this study is to observe the link between plasma level and urinary excretion of each and changes during acute exacerbation. Method: Plasma and 24 hour urine were collected from sixteen asthmatics during acute exacerbation, twice; first day of symptomatic exacerbation and two weeks after treatment. Controls were ten healthy normal subjects. All patients were treated with corticosteroid and beta-2 adrenergic agonist on admission. ET was determined by radioimmunoassay and had 100% cross reactivity with ET-1, 67% with ET-2, 84% with ET-3, and 8% with Big-ET. Results: Plasma ETs were significantly elevated during acute attack of asthma compared with those in remission and controls. However, there was no significant changes in urine ET concentrations or total ET amounts in 24 hour urine during exacerbation up to two weeks. Those levels of urine ET in asthmatics were still higher than controls. ET concentrations in plasma or urine were not correlated with pulmonary functional parameters and hypoxemia. Conclusion: The findings suggests that increased plasma ETs are related with exaggerated release during acute asthma. Urinary ET excretion is increased in asthma. However, urine ET changes during exacerbation should be observed in a larger and longer scale.