The estrogen receptor content, determined immunohistochemically by staining with monoclonal antibodies, was studied in seven consecutive cases of endometrial stromal sarcoma. In addition, the mitotic rate and immunohistochemical patterns (vimentin, desmin, actin, cytokeratins) were determined. Five of seven cases contained estrogen receptors, three low-grade (fewer than 10 mitoses/10 high-power fields [hpf]) and two high-grade (10 or more mitoses/10 hpf), the latter having a relatively low mitotic rate. The two negative cases were high-grade with a very high number of mitoses. Tumor cells were positive only to vimentin. Based on these results, we postulate that estrogen receptors are undetectable only in highly proliferating neoplasms. Thus, immunohistochemically determined estrogen receptor levels may indicate the aggressiveness of endometrial stromal sarcoma, and might be a guideline in selecting patients for hormonal therapy. The presence of positive immunostaining to vimentin confirms the mesenchymal origin of endometrial stromal sarcoma.