We occasionally encountered clear cell adenofibromatous (CCAF) components coexisting in the ovarian clear cell adenocarcinoma (CCA). To reveal the clinicopathologic significance of CCAF components in CCA, we classified 67 cases of surgically resected CCA into CCA with and without CCAF components [CCAF(+) and (-) groups], and compared clinicopathologic parameters, that is, patient age, clinical stage, the degree of optimal cytoreduction, patient outcome, histologic grade and Ki-67 labeling index of the CCA, and the presence of endometriosis, between these 2 groups. Fourteen cases (21%) and 53 cases were classified as CCAF(+) and CCAF(-) groups, respectively. Of these 14 CCAF(+) cases, the CCAF components with atypia were observed adjacent to the CCAF components without atypia in 10, and adjacent to the obvious CCAs in 13 cases. In comparison with the CCAF(-) group, the CCAF(+) group showed a higher frequency of histologically low-grade tumors [93% (13 of 14) vs. 43% (23 of 53), P=0.0027], a lower Ki-67 labeling index (mean 35.9% vs. 44.0%, P=0.0492), and better patient prognosis (5-year survival 78.8% vs. 49.3%, P=0.0277). Endometriosis was much less frequent in the CCAF(+) group than in the CCAF(-) group [14.7% (2 of 14) vs. 67.9% (36 of 53), P=0.00096]. Multivariate analysis identified only optimal cytoreduction as independent favorable prognostic factor. These results suggest that CCAF besides endometriosis is associated with the development of CCA, and that the CCAF(+) group may be a distinct subgroup of CCA with less aggressive biologic behavior.