To determine the immunohistochemical staining profile of endometrial stromal cells, we analyzed a series of formalin-fixed, paraffin-embedded normal endometrial tissues, stromal nodules, and stromal sarcomas for immunoreactivity with a panel of eight antibodies. Normal proliferative-phase (five cases) and secretory-phase (five cases) endometrial stromal cells showed the following immunopositivity: vimentin 10 of 10, muscle-specific actin (MSA) 10 of 10, alpha-smooth muscle actin (alpha sm) 10 of 10, desmin nine of 10, cytokeratin (AE1/AE3 and CAM 5.2) zero of 10, epithelial membrane antigen (EMA) zero of 10, and S-100 protein zero of 10. Antibodies to vimentin, MSA, and alpha sm stained a greater number of proliferative-phase stromal cells as compared with secretory-phase cells. Only rare stromal cells were immunoreactive for desmin, except for one case in which predecidual cells were diffusely positive. Both endometrial stromal nodules reacted with antibodies to MSA, alpha sm, and desmin, and one was vimentin positive. Each was unreactive for epithelial markers and S-100 protein. The 12 endometrial stromal sarcomas had the following immunopositivity: vimentin 11 of 12, MSA 10 of 12, alpha sm 10 of 12, desmin seven of 12, AE1/AE3 one of 12, CAM 5.2 two of 12, EMA zero of 12, and S-100 protein zero of 12. The antibodies to MSA and alpha sm usually stained a greater number of cells than did the desmin antibody. Three stromal sarcomas had sex cord-like areas, one of which exhibited focal CAM 5.2 positivity. These immunohistochemical findings for normal and neoplastic endometrial stromal cells indicate smooth muscle differentiation and are similar to those of smooth muscle neoplasms and myofibroblastic cells.