According to recent studies showing that human papillomavirus (HPV) infections can be influenced by sex steroid hormones, we performed estrogen (ER) and progesterone (PgR) receptor assays in fresh frozen biopsies of genital-HPV-related lesions. Seventy-three women with normal cervix, condyloma, low- and high-grade CIN and squamous carcinoma were evaluated in comparison with 15 persons with vulvar and 9 with penile papillomavirus-associated lesions. HPV genotypes were determined by dot-blot hybridization. Non-cervical lesions did not express HR. Condyloma on squamous metaplasia of the cervix and high-grade CIN expressed high levels of HR, particularly PgR (mean 4,086 and 4,518 fmoles/g tissue, respectively). Cervical squamous carcinoma expressed very low concentrations of PgR in a limited number of cases. High levels of PgR were correlated with high-grade CIN (p < 0.05), HPV16-18-associated lesions (p < 0.01) and ER were correlated to HPV6-11-related lesions (p < 0.01). The levels were independent of age, cycle stage and oral contraception. Morphological localization of PgR, using an immunocytochemical method using a monoclonal antibody (MAb) (PR-ICA), showed intense homogeneous staining in the nuclei of the stromal fibroblasts underlying dysplastic epithelium and condyloma on squamous metaplasia. These results suggest that, under in vivo conditions, sex steroid hormones, particularly progesterone, may act indirectly on HPV-infected epithelial cells and be implicated as co-factors in HPV-related cervical neoplasia. They could explain the relative predisposition to malignant transformation of the cervix as compared with vulvar and penile mucosa.