Biochemical study of fibroepithelial tumors of the female breast showed presence of progesterone receptor (PgR) in all five cystosarcoma phyllodes (two malignant, three benign), and in 11 of 13 fibroadenomas tested. Estrogen receptor (ER) was detected in only one of five cystosarcomas and 2 of 13 fibroadenomas. The relative volumes occupied by epithelium and stroma in each tumor were measured from histologic sections. The results were consistent with presence of PgR in the stroma and ER in the epithelium. Different types of cystosarcoma (benign and malignant) and different types of fibroadenomas (intracanalicular, pericanalicular, and mixed) did not differ significantly in content of PgR, and mean levels of PgR in cystosarcoma were comparable with those in fibroadenomas. The presence of PgR in cystosarcomas suggests that progestational therapy, and possibly other forms of hormonal therapy, should be tested in the treatment of advanced, malignant cystosarcoma phyllodes.