The success of a prevention program on a cytological basis depends above all on the localization and extent of the nuclear changes, both of which can be determined using simple morphological methods. Using histologic specimens taken from the cases of hyperplasias at the Erlangen University Gynecological Hospital between 1963 and 1981, location of the endometrial hyperplasias and nuclear pattern were determined. It was established that 8.6% of precancerous endometrial hyperplasias are situated exclusively in deeper regions of mucous membrane and thus, as a rule, escape detection by cytologic instruments. No nuclear enlargement was shown in 22.9% of precancerous cells. 14.2% showed no changes in chromatin patterns, and 8.6% of precancerous cells revealed neither nuclear enlargement nor changes in chromatin. Since a number of precancerous cells are situated in deeper sections of mucous membrane and fail to show nuclear enlargement, cytologic screening cannot provide a basis for successful recognition of the precancerous stage of endometrial carcinoma.