Melanocytic nevi excised from women who were pregnant or had given birth less than 1 month previously or from women who were taking oral contraceptives were studied with the use of a fluorescent estrogen and progesterone ligand binding assay. All nevi from these patients had markedly increased numbers of estrogen- and progesterone-binding cells, similar to those observed in melanoma and melanoma precursor lesions. In contrast, nevus cells in acquired intradermal nevi from women who were not pregnant nor taking oral contraceptives had few or no detectable estrogen- or progesterone-binding cells. The induction of increased hormonal receptors by pregnancy or oral contraceptives may be related to an increased risk of melanoma and melanoma precursor lesions in specific subsets of patients, as well as to the pigmentary changes observed in female patients in general.