We attempted to clarify the relationship between circulating endothelin-1 levels and bronchial hyperreactivity in asthmatic patients. Sixty-two asthmatic patients were studied for plasma endothelin-1 levels during acute asthma. The relationship between plasma endothelin-1 levels and pulmonary function (FEV1/FVC) was examined in 17 patients. Twelve patients were studied for changes in plasma endothelin-1 levels during methacholine inhalation tests. Plasma endothelin-1 levels during acute asthma correlated with the severity of acute asthma defined with the Clinical Asthma Evaluation Score, and those during the methacholine challenge test correlated with the grade of bronchoconstriction (P < .01). A significant negative correlation was observed between plasma endothelin-1 levels and FEV1/FVC ratio (P < .01). These results suggest that circulating endothelin-1 levels may reflect the severity of acute asthma.