Androgen receptor (AR) activity is required for prostate growth, differentiation, and secretion. Deregulation of AR activity results in inappropriate mitogenic signaling and is thought to contribute both to the initiation and progression of prostate cancers. Cyclin D1 functions as a strong AR corepressor by directly interacting with and inhibiting receptor activity. However, the extent to which cyclin D1 functions to inhibit AR activity under conditions associated with cancer progression has not been determined. We now demonstrate that cyclin D1 action is conserved in multiple tumor cell backgrounds, inhibiting AR-dependent gene activation in breast, bladder, and androgen-independent prostatic adenocarcinoma cell lines. In androgen-dependent prostatic adenocarcinomas, cyclin D1 effectively muted androgen-stimulated target gene expression in a manner analogous to dominant negative ARs. The ability of cyclin D1 to inhibit AR activity was conserved with regard to target promoter, repressing transactivation from mouse mammary tumor virus, probasin, and prostate-specific antigen promoters. Inappropriate, nonligand AR activation, postulated to act through regulation of receptor phosphorylation, was also sensitive to cyclin D1 regulation. Moreover, we show that several phosphorylation site mutants of the AR were equally inhibited by cyclin D1 as compared with the wild-type receptor. Given these data establishing the potency of cyclin D1-mediated repression, we evaluated the ability of cyclin D1 to inhibit tumor-derived AR alleles and polymorphisms associated with tumor progression and increased prostate cancer risk. We demonstrate that the AR alleles and polymorphisms tested respond completely to cyclin D1 corepressor activity. In addition, activation of a common tumor-derived AR allele by 17β-estradiol and progesterone was inhibited through ectopic expression of cyclin D1. Taken together, these data establish the potency of cyclin D1 as an AR corepressor and provide support for additional studies examining the efficacy of developing novel prostate cancer therapies for both androgen-dependent and -independent tumors.