Endothelin-1 (ET-1) is a novel potent vasoconstrictor peptide discovered in the supernatant fraction of cultured endothelial cells. We measured plasma levels of ET-1 using a sensitive sandwich enzyme immunoassay. Plasma concentrations of ET-1 in 31 patients with systemic sclerosis (SSc) (1.90 +/- 0.47 pg/ml) were higher than those (1.31 +/- 0.10 pg/ml) in 25 age and sex matched healthy subjects. Patients with SSc with diffuse scleroderma had higher levels of ET-1 compared with those with limited scleroderma. Plasma ET-1 levels correlated inversely with carbon monoxide diffusing capacity (DLco). Measurement of plasma ET-1 levels may be useful as a predictor of prognosis of SSc.