Aromatase expression has been described in stromal cells of endometriosis, adenomyosis and endometrial cancer. We analyzed aromatase expression in a series of 23 low-grade endometrial stromal sarcomas. Archival formalin-fixed and paraffin-embedded material was analyzed with immunohistochemistry. Aromatase expression was evaluated with a monoclonal and a polyclonal antibody using the peroxidase-antiperoxidase method. A score was calculated based on the percentage of positive tumor cells and the staining intensity. Aromatase was seen in 19 (83%) of 23 tumors with monoclonal antibody and 20 (87%) of 23 tumors with polyclonal antibody. Aromatase expression using the monoclonal antibody was scored as high in five (22%), moderate in nine (39%) and low in five (22%) tumors. Four (17%) low-grade endometrial stromal sarcomas did not stain for aromatase. Aromatase expression with the polyclonal antibody was scored as high in seven (31%), moderate in four (17%) and low in nine (39%) tumors. Three (13%) low-grade endometrial stromal sarcomas did not stain for aromatase. Little or no aromatase expression tended to correlate with stage I disease, while higher scores were more frequently associated with advanced disease. Our results demonstrate that most low-grade endometrial stromal sarcomas express aromatase. The staining pattern, however, is heterogeneous. The high percentage of aromatase positivity in low-grade endometrial stromal sarcomas may have implications in the management of these tumors and offer new treatment modalities such as hormonal therapy with aromatase inhibitors.