Several types of mural nodules may develop in the wall of mucinous tumors of the ovary. The histopathologic features and prognosis of foci/nodules of anaplastic carcinoma are not well known. Although they were first thought to carry an invariably unfavorable prognosis, recent data indicate that this does not necessarily apply to stage Ia tumors. Slides from 34 consultation cases of mucinous ovarian tumors with foci/nodules of anaplastic carcinoma were reviewed and classified on the basis of their morphologic features. Cytokeratin stains were done in selected cases. Staging, treatment, and follow-up information were obtained. The foci/nodules of anaplastic carcinoma were classified histologically into 3 groups: (a) rhabdoid (n=12) having a diffuse arrangement of cells with large, bright, eosinophilic cytoplasms, eccentric nuclei, and one or more prominent nucleoli; (b) sarcomatoid (n=10) characterized by a spindle cell proliferation, with atypical and vesicular nuclei often with herringbone pattern; and (c) pleomorphic (n=12) exhibiting overlapping features of the first 2 categories. International Federation of Gynecology and Obstetrics (FIGO) stage was available in 31 cases: 15 were stage Ia, 6 stage Ic, 2 stage II, 7 stage III, and 1 stage IV. Follow-up was obtained in 21 cases. Seven patients died of the disease after a median time of 8 months: 3 stage IC, 1 stage II, 1 stage III, 1 stage IV, and the other was unstaged. Ten patients were alive and clinically free of disease after a median follow-up of 5 years: they were all stage Ia. Three patients (2 stages III and 1 stage IV) were alive with disease at 3 years, 9 months, and 7 months. The other patient (stage Ia) died of an unrelated cause. Thus, only 1 patient with stage Ia disease died, and she died of causes other than ovarian cancer. The presence of foci/nodules of anaplastic carcinoma in unruptured stage I mucinous tumors of the ovary does not necessarily carry an adverse prognosis. These foci/nodules may exhibit rhabdoid, sarcomatoid (spindle cell), or pleomorphic features.