In a period of 5 years 116 specimens were collected with various techniques from patients with the homologous type of cystic-glandular hyperplasia of the endometrium. These were systematically studied with a view to identifying a potential progesteron effect on the morphology of the endometrial and, given sufficient material, cervical mucosa. While the endometrium was examined for discrete areas with signs of secretion, attention in cervical specimens was directed to the potential presence of focal alveolar proliferations of cervical glands with basal cell hyperplasia (socalled cribriform polypoid hyperplasia). Progesteron-induced changes of major endometrial areas were found to be present in 6,9% of cases; the "cribriform polypoid hyperplasia" involving the cervical mucosa was seen in 9 of 43 cases (20,9%) with adequate cervical specimens. In 2 of the patients with progesterone-induced cervical changes luteinized follicular epithelium in the ovaries was indicative of effective endocrine activity. The different action of progesterone on the endometrial versus the cervical mucosa is discussed on the basis of katamnestic studies.