The proliferation of squamous cells of the vagina and cervix uteri is induced by steroid hormones during the menstrual cycle. However, carcinoma of the cervix cannot be influenced by any hormone therapy. Forty-four different cervical specimens (different days in the menstrual cycle of healthy women and those with dysplastic lesions and carcinomas of the cervix) have been tested for estrogen (ER) and progesterone (PR) receptor protein content by means of immunohistochemistry. The ER content of the squamous epithelium depends upon the menstrual cycle: in the early proliferative phase cells of all layers are negative. In the midphase of proliferation the basal and parabasal layers become positive, and in the secretory phase positive cell nuclei can be found up to the superficial layers. A weak reaction to ER staining is found only in mild dysplastic lesions of the uterine cervix; severe dysplastic forms and invasive carcinomas were all negative. No positive PR was found in any squamous cell tissue. Stroma cells of the uterine cervix showed different straining intensity for ER and PR, regardless of the menstrual cycle. The loss of ER in the neoplastic cell could be an explanation for three clinical experiences: premenopausal patients have no tumor progression of the cervix uteri despite normal ovarian function; the duration of survival shows no relation to the receptor status of cervical carcinomas; and antihormonal treatment of cervical carcinomas produces no appreciable therapeutic success.