Endothelin (ET)-1 concentration was measured in tissue extracts from 157 human breast cancers, 26 benign breast tumors and 10 normal breast tissues. ET-1 was detectable in all tissue extracts by specific radioimmunoassay, the concentration ranging from 0.12 to 11.95 pg/mg protein. ET-1 concentration was significantly higher in tissue extracts from breast cancer than in those from benign breast tumor or normal control tissue, suggesting that this peptide is synthesized by breast cancer cells. When 157 breast cancer specimens were categorized into 4 groups in terms of clinical staging, no significant difference was found among these clinical staging groups. Furthermore, no significant associations were observed between ET-1 concentrations and the age of patients, tumor size, lymphnode involvement, histological type or steroid hormone receptor status.