Mullerian adenosarcomas (MAs) are rare mixed mesenchymal and epithelial neoplasms that occur most commonly in the uterus. Although the epithelial component is typically benign, the mesenchymal component of most adenosarcomas morphologically resembles that observed in endometrial stromal tumors and is responsible for their clinical behavior. Thus, the differential diagnosis usually includes not only low-grade endometrial stromal tumors, but also adenofibroma, carcinosarcoma, and embryonal rhabdomyosarcoma especially in small samples. The objective of this study was to ascertain the immunophenotypic profile of the epithelial and mesenchymal components of MAs and delineate possible differences between conventional mesenchymal areas and areas of sarcomatous overgrowth. Representative sections from 35 MAs, 28 of them without sarcomatous overgrowth (MA-NSO) and 7 with sarcomatous overgrowth (MA-SO), were included in the study. Thirty tumors arose in the uterus, 4 were pelvic, and 1 originated in the colon. Adequate blocks were selected and immunostained for estrogen receptor (ER), progesterone receptor (PR), androgen receptor (AR), CD10, WT1, smooth muscle actin, desmin, AE1/3 cytokeratin, CD34, calretinin, inhibin, c-kit, and Ki-67. The mesenchymal component expressed ER in 21/27 MA-NSOs but in only 1/7 MA-SOs (65% overall). PR was expressed in 21/26 MA-NSOs and 4/7 MA-SOs (76% overall), whereas AR was positive in 10/27 MA-NSOs and 5/7 MA-SOs (35% overall). CD10 was expressed in 23/28 MA-NSOs but in only 2/7 MA-SOs (71% overall), and WT1 positivity was seen in 22/27 MA-NSOs and 6/7 MA-SOs (79% overall). Sixty-seven percent of MAs expressed smooth muscle actin, 32% desmin, including both examples of MA-SOs with rhabdomyoblastic differentiation, and 25% expressed AE1/3 cytokeratin. CD34 expression was found in 35% of the tumors, but it was almost always patchy in distribution and weak in intensity, as was calretinin expression, seen only in 12% of the cases. Expression of c-kit and inhibin in greater than 5% of the tumor cells was not encountered. The median and mean Ki-67 labeling indices were 10% and 12%, respectively (range, <5% to 40%). The median and mean Ki-67 indices were both 5% in MA-NSOs compared with 30% and 28%, respectively, in MA-SOs. The epithelial compartment demonstrated expression for ER (24/32), PR (23/31), and AE1/3 cytokeratin (33/33); rare cases expressed CD10 (4 cases) and AR (1 case). In summary, the immunophenotype of most MAs resembled that of endometrial stromal tumors (positive for ER, PR, WT1, and CD10, with variable expression of muscle markers, AR and cytokeratin). The proliferative rate in the stromal component was strongly related to the presence of sarcomatous overgrowth. ER, PR, and CD10 expression was lost in MA-SOs relative to conventional low-grade stromal areas of mullerian/mesodermal adenosarcomas, reflecting the "dedifferentiation" of this component.