Desmoid tumors are benign fibrous neoplasms which arise from the fibrous tissue of intra- and extra- abdominal sites, but their clinical management is sometimes difficult because of extensive infiltration into the surrounding tissues. Desmoid tumors commonly occur in women, especially after childbirth. Recently, both clinical and experimental findings indicate the possible roles of sex steroids in the development and progression of desmoid tumors but detailed information is still ambiguous. In this study, we first examined immunoreactivity of sex steroid receptors in desmoid tumors (27 cases) by immunohistochemistry and compared the findings with those in reactive self-limiting lesions associated with fibrosis (8 cases). Estrogen receptor (ER) α and ERβ immunoreactivities were detected in 7.4% (2/27) and 7.4% (2/27) of desmoid tumors, respectively. One desmoid tumor expressed both ERα and ERβ. Progesterone receptor (PR)-A and PR-B were detected in 25.9% (7/ 27) and 33.3% (9/27), respectively, and androgen receptor (AR) in 52.9% (14/27). In reactive lesions with fibrosis, only AR was detected in 37.5% (3/8). Sex steroid receptor mRNAs was further examined by reverse transcription and polymerase chain reaction (RT-PCR) analysis using fresh frozen tissues, demonstrating the expression of PR (PR-A and/or PR-B) and AR mRNAs in eight desmoid tumors examined and all cases of reactive fibross. These results indicate that sex steroid hormones might play an important role in the pathogenesis of desmoid tumors and could lead to the introduction of novel hormone therapeutic approaches in managing patients with recurrent desmoid tumors.