Purpose: Changes in fatty acid synthase levels were investigated in the CWR22 xenograft model of prostatic adenocarcinoma after castration and during progression to androgen independence. Materials and Methods: Male nude mice with CWR22 tumors were castrated and sacrificed 3, 7, 21, 28 or 42 days after castration. Some mice received testosterone replenishment 21 days after castration and were sacrificed 7 days later. In addition, other castrates were maintained for 7 to 10 months to allow tumors to relapse to androgen independence. Fatty acid synthase was measured by immunohistochemical study and Western blot analysis. Results: Fatty acid synthase decreased rapidly after castration and after 28 to 42 days it was 5% to 10% of the level in tumors of intact controls. Temporal changes in fatty acid synthase after castration were associated with decreased proliferative potential and increased levels of the cyclin dependent kinase inhibitor p27Kip-1. Testosterone treatment of castrates at 21 to 28 days after castration increased fatty acid synthase expression to the level in tumors of intact controls. Tumors of long-term castrates with post-castration androgen independent growth had increased fatty acid synthase, as did small focal areas of androgen independent proliferation in tumors that had not increased in size by 7 to 10 months. In relapsed CWR22 tumors and in focal areas of androgen independent proliferation in nonrelapsed CWR22 tumors increased fatty acid synthase was associated spatially with increased proliferation and decreased p27Kip-1. Conclusions: Fatty acid synthase in CWR22 tumors depends initially on testosterone and it is associated with androgen mediated proliferation. Furthermore, increased fatty acid synthase levels associated with androgen independent proliferation may represent an early event in the development of the androgen independent phenotype.