The pattern of estrogen (ER) and progesterone receptors (PR) and their relationship to histo- and cyto-pathological parameters has been studied in 97 cases of benign breast disease and benign phyllode tumors (95 women, of whom 76 were premenopausal, and 2 men). Total (cytosolic + nuclear) ER and PR were assayed by a single-saturating dose method using a tris-KCl buffer. The cut-off between positive and negative ER and PR assay was 100 femtomoles/g tissue. All specimens were processed for histological examination: epithelial and fibroblastic proliferation, epithelial/stromal ratio and presence of focal or diffuse hyalinosis. In 33% of the 46 cases of fibrocystic disease one receptor at least was present (13% ER+, 31% PR+). All the 8 cases in which infiltrating epitheliosis was present were PR+ and 4 of them were also ER+. In 72% of the 31 fibroadenomas one receptor at least was present (19% ER+, 71% PR+). In all these cases levels of receptors were lower than in malignant tumors. An inverse correlation between PR + prevalence and fibrohyalinosis was observed; on the other hand a positive relationship between PR + and fibroblastic (p less than 0.001) or epithelial (p less than 0.01) proliferation was found. In all 5 benign phyllode tumors examined PR + were present at a very high level, almost as high as in malignant tumors. Of the 15 other benign breast lesions, all but one (1 hamartoma) were ER- and PR-.