A large, consecutive series of 562 patients with endometrial adenocarcinomas was investigated with respect to prognostic factors. In the histopathologic evaluation the World Health Organization (WHO) classification system was used. In addition to that, in moderately differentiated (MD) tumors small areas of solid growth were identified and the proportions of these out of the whole areas of tumor (in the predominant number of cases this being less than 5%) were later determined by morphometry, showing a good accordance with the subjective estimations. Differentiated tumors with small solid areas (MD + S tumors) implied a significantly worse prognosis compared to tumors without a solid component (P < 0.001), which was also confirmed in a multivariate analysis. In the multivariate analysis MD+S differentiation had an independent prognostic impact, as strong as age, clinical stage and myometrial invasion. It is suggested that the occurrence of even a very small solid component is an ominous sign, the presence (or absence) of which might be an important parameter to take into consideration in the discrimination between high- and low-risk endometrial carcinomas.