Endothelins (ETs) are a family of peptide mediators that have a number of biological properties, including the ability to act as potent bronchoconstrictors of isolated human airways. To examine the possible involvement of ET in asthma, we have performed fiberoptic bronchoscopy and bronchoalveolar lavage (BAL) on 10 healthy control subjects, 10 patients with atopic asthma treated with bronchodilators alone, and 8 patients with atopic asthma treated with inhaled and/or oral corticosteroids. Endothelin concentrations in BAL fluid were measured by radioimmunoassay and total protein concentrations by a colorimetric method. There was a significant increase in the BAL fluid ET levels in the non-steroid-treated patients with asthma compared with the normal subjects, when expressed either as a concentration (median, 0.30 versus 0.08 pM; p = 0.001) or in relation to total protein (median, 3.02 versus 1.08 pmol/g; p = 0.01). There was, however, no statistically significant difference in ET levels between the steroid-treated patients with asthma, and either of the other two groups. In the non-steroid-treated patients with asthma there was a significant negative correlation between the BAL fluid ET concentration and the % predicted FEV1 (r = -0.71, p = 0.03). This correlation was not significant in the steroid-treated subjects, and no correlation between BAL fluid ET concentrations and bronchial reactivity was found in any of the three groups. These findings are consistent with the hypothesis that ET contributes to the pathophysiology of asthma.