Ten endometrial stromal tumors of the uterus with a prominent myxoid or fibrous appearance, or both, that led to problems in interpretation are reported. The patients were 32 to 52 (mean 39) years of age. Three presented with dysfunctional uterine bleeding and one with abdominal pain. An enlarged uterus or a pelvic mass was palpated in five patients; the tumor was an incidental postpartum finding in one patient. All patients underwent hysterectomy. The tumors ranged from 4 to 20 cm in greatest dimension. Six were soft, polypoid intracavitary masses and four were predominantly intramyometrial; two were gelatinous. On microscopic examination, nine tumors infiltrated the myometrium (stromal sarcomas) and one was well circumscribed (stromal nodule). Six tumors had a predominantly fibrous component with the neoplastic cells separated by variable amounts of collagen; extensive areas of hyalinization were present in three tumors. Two tumors were predominantly composed of hypocellular areas with an abundant myxoid matrix, and two had both components in roughly equal proportions. Alcian blue staining was positive, with the staining eliminated by hyaluronidase predigestion, in the myxoid areas. The typical morphologic features of endometrial stromal neoplasia were present focally in four tumors. All of them contained numerous small thin-walled vessels. Vimentin and smooth muscle actin were positive in nine of nine and seven of nine tumors, respectively, whereas desmin was negative in six of nine tumors and only focally positive in the other three. One patient had omental nodules at the time of the initial diagnosis and another had a pelvic recurrence 2 years after hysterectomy. Follow-up information is unavailable or short in the other cases. These tumors should be considered of endometrial stromal origin in view of the typical location of most of them, their growth pattern, content of characteristic arterioles, presence of typical endometrial stromal neoplasia in the primary or recurrent tumor in some cases, and absence of evidence of origin from a cell type other than endometrial stroma. These tumors may be identical, in some instances at least, to tumors referred to in the older literature as "myxofibrosarcomas."