The presence of clear cells in genital tract neoplasms often reflexly prompts the diagnosis of a clear cell tumor but clear cells may be seen in many other neoplasms. In one such, those of endometrioid type, they are well known and generally readily characterized when they have the secretory pattern that recapitulates the well-known morphology of early secretory endometrium. In this report, we describe various clear cell morphologies, some possibly related to the secretory variant, but others not, and all potentially the source of significant diagnostic difficulty. We reviewed 21 endometrioid tumors that occurred in patients from 27 to 88 (median 64) years. Most had an adnexal mass (13) or abdominal swelling (4), but 4 presented with vaginal bleeding. One tumor formed a cystic mass in the pelvis, 1 tumor involved the right fallopian tube, 1 the endometrium, and 18 the ovary. One tumor was a cystadenofibroma, 1 a borderline tumor, and 19 were adenocarcinomas. Twelve patients had stage I, 4 stage III, 1 stage IV, and 4 were unstaged. One patient who had a previous hysterectomy and salpingo-oophorectomy underwent resection of a cystic pelvic mass; others all were treated with abdominal hysterectomy and salpingo-oophorectomy. All the neoplasms had the typical architecture of endometrioid tumors but differed markedly in their cytoplasmic features. In 18 of the tumors at least one-third of the cells had clear cytoplasm and 3 had only clear cells. The clear cytoplasm varied from foamy to empty and the nuclei had a variable location, basilar, central, and apical. The clear cells were associated with squamous differentiation in only 1 case in which the change appeared hydropic. By immunohistocytochemistry, the clear cells were focally positive for epithelial markers in most cases but in some cases one or more of these immunostains were negative. Tubulocystic, solid, and papillary patterns of clear cell carcinoma were absent. Periodic acid-Schiff was focally positive for glycogen in the cytoplasm in 5 cases. Although a few cases had a focal slight resemblance to secretory endometrioid carcinoma most did not and the orderly morphology of classic secretory carcinoma was noteworthy for its absence. The nature of the cytoplasmic clarity is usually uncertain but is likely variably owing to lipid, mucin or glycogen accumulation, or is a hydropic change. The distinction from clear cell carcinoma depends on awareness of this unusual variant of endometrioid neoplasia and a lack of the distinctive patterns of clear cell carcinoma; at this time, special studies, including immunohistochemistry, do not aid significantly although certainly negative reactions, such as for thyroglobulin protein (arguing against clear cell struma ovarii) may play a role in some differential diagnostic considerations. There are prognostic and therapeutic implications in the distinction with clear cell carcinoma.