Immunoreactive endothelin 1 (irET-1) concentrations were measured in extracts prepared from 4 phyllodes tumors and 14 fibroadenomas. irET-1 was detectable in all tissue extracts by specific radioimmunoassay, and the mean concentration of irET-1 was 18-fold and 27-fold higher in tissue extracts from phyllodes tumors than in those from intracanalicular fibroadenomas and pericanalicular fibroadenomas, respectively. Reverse-phase high-performance liquid chromatography coupled with radioimmunoassay in the extracts from phyllodes tumors revealed one major irET-1 component corresponding to human standard ET-1. Furthermore, immunocytochemical staining for ET-1 revealed that numerous ET-1-immunoreactive cells were seen in the epithelial cells but not in the stromal cells, suggesting that ET-1 is synthesized by the epithelial component of phyllodes tumors. A possible paracrine role of ET-1 in the growth of this rare tumor which is characterized by its prominent stromal cellularity is discussed.