A clinicopathologic study of uterine endometrial stromal tumors (EST) has been performed with special emphasis on histologic and immunohistochemical differential criteria and prognostic factors. The material comprised three stromal nodules (SN), twelve low grade stromal sarcomas (LGESS) and five high grade stromal sarcomas (HGESS). Previously unreported endolymphatic growth was found within one SN. EST showed an association of mitotic index (IM) with atypia, degree of stromal differentiation, additional non-stromal differentiation and venous invasion. IM was the best criterion in the differential diagnosis of LGESS and HGESS and the most significant histologic prognosticator. The present study shows that a histologic grade of stromal sarcoma was a more significant prognostic factor than pTNM stage. The results suggested that clinicopathologic classification of EST could be supplemented by including the following subgroups: a) SN--without intravascular growth, and potentially malignant SN--with endolymphatic growth within the tumor; b) LGESS with IM < 2 and no atypia, and LGESS with 2 > IM < 10 and mild atypia; c) HGESS with 10 > IM < 20 and moderate atypia, and HGESS with IM > 20 and marked atypia. Contrary to common view these observations indicate that the distinction of some SN and LGESS from stromal hyperplasia is possible in an endometrial curretage material.