Ovarian myxomas recently have been reported as new, distinct pathologic entities that show a myxoid, moderately cellular proliferation of spindle and stellate cells interspersed with areas of fibrosis, hemorrhage, and delicate vascular spaces. These histologic features are frequently seen in the thecoma-fibroma group of ovarian stromal tumors. For this reason, we propose that ovarian myxomas are part of the spectrum of differentiation in thecomas-fibromas of the ovary. To provide histologic and immunohistochemical evidence for this proposal, four ovarian myxomas were compared with 48 primary ovarian stromal tumors in the thecoma-fibroma group from 46 patients. The thecoma-fibroma group of stromal tumors included 23 thecomas, 23 fibromas, and two sclerosing stromal tumors. We found significant (> 25% of histologic appearance) myxoid change in six thecomas and one sclerosing stromal tumor. This myxoid change resembled the histologic appearance of an ovarian myxoma. Immunohistochemical studies on paraffin-embedded material showed vimentin immunostaining in all tumors. Smooth-muscle actin was present in all of the myxomas, in two of the two sclerosing stromal tumors, and in 20 (90%) of the 23 thecomas, but it was present in only 11 (48%) of the 23 fibromas. Desmin staining was not present in any of the four ovarian myxomas or in the two sclerosing stromal tumors, and only three (13%) of the 23 thecomas showed focal staining for desmin. Nine (39%) of the 23 fibromas expressed desmin. S100 protein was expressed in one fibroma and one thecoma, weakly. None of the ovarian myxomas or the thecoma-fibroma group of stromal tumors expressed cytokeratins as detected by three different monoclonal antibody cocktails, ie, cytokeratin AE1/AE3, cytokeratin CAM 5.2, or cytokeratin MAK-6. The ovarian thecoma-fibroma group of stromal tumors form a histologic spectrum of lesions in which clear-cut distinguishing points between various entities are difficult to define. The myxoid change, present in the thecoma-fibroma group of tumors, was indistinguishable histologically and immunohistochemically from ovarian myxoma. For this reason, we propose that ovarian myxomas may be at one end of the spectrum of differentiation in the thecoma-fibroma group of tumors, in which no remaining stromal tumor is detectable.