Endometrial hyperplasias and carcinomas have been investigated by quantitative microscopical methods. The aim of this study was to develop an objective method of histopathological diagnosis because of the great subjective variability of these endometrial conditions. A total of 79 cases were studied. Only those were included, which, after re-examination of the material by different pathologists, were regarded as mild atypical hyperplasia (38 cases), marked atypical (adenomatous) hyperplasia (11 cases), well and moderately differentiated adenocarcinoma (19 and 11 cases, respectively). There are many significant differences between these four groups. Stereological features contained the most important information, whilst nuclear features were less successful discriminators. The volume percentage of the epithelium and of the glands and the inner surface density of the glands showed the most significant differences between adenomatous hyperplasia and well differentiated carcinoma. On the basis of these findings, a quantitative model is suggested which might give more insight into the development of these abnormalities, assuming a gradual transition from one to another. The great advantage of the quantitative analysis of data over subjective impressions, it that is is objective, and provides consistently reproducible results. The present results may be especially useful when histopathological diagnosis is in doubt. It is concluded that application of quantitative microscopy in the discrimination of endometrial hyperplasia and carcinoma in histopathology is feasible.