Huntingtin-interacting protein 1 (HIP1) is frequently overexpressed in prostate cancer. HIP1 is a clathrin-binding protein involved in growth factor receptor trafficking that transforms fibroblasts by prolonging the half-life of growth factor receptors. In addition to human cancers, HIP1 is also overexpressed in prostate tumors from the transgenic adenocarcinoma of the mouse prostate (TRAMP) mouse model. Here we provide evidence that HIP1 plays an important role in mouse tumor development, as tumor formation in the TRAMP mice was impaired in the Hip1null/null background. In addition, we report that autoantibodies to HIP1 developed in the sera of TRAMP mice with prostate cancer as well as in the sera from human prostate cancer patients. This led to the development of an anti-HIP1 serum test in humans that had a similar sensitivity and specificity to the anti-alpha-methylacyl CoA racemase (AMACR) and prostate-specific antigen tests for prostate cancer and when combined with the anti-AMACR test yielded a specificity of 97%. These data suggest that HIP1 plays a functional role in tumorigenesis and that a positive HIP1 autoantibody test may be an important serum marker of prostate cancer.