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Five cases of pseudoangiomatous hyperplasia of mammary stroma, together with seven examples of mammary hamartoma, were probed with monoclonal antibodies H222 and KD68 to investigate the possible role of estrogen and progesterone receptor expression in the pathogenesis of these benign stromal proliferations. All five cases of pseudoangiomatous hyperplasia showed patchy, intense labelling of the stromal cells with progesterone receptor antibodies, a pattern contrasting markedly with the absence of immunoreactivity in normal (nongestational) mammary stroma or the stromal component of common juvenile mammary hyperplasia. The stroma of the hamartoma group labeled inconsistently, with the notable exception of three myoid hamartomas. Stromal immunoreactivity was diffuse and intense in two of these, and patchy and distinct in the remaining case. These findings (a) support the contention that pseudoangiomatous hyperplasia represents a localized form of stromal overgrowth with a hormonal (primarily progestagenic) etiology and (b) further highlight the heterogeneity of socalled mammary hamartomas by demonstrating dramatically different progesterone receptor immunoreactivity patterns in myoid lesions as compared with other hamartoma variants.