Prostasomes are secretory granules synthesized, stored and secreted by normal and neoplastic human prostate epithelial cells and by prostate cancer metastasis. Prostasomes are postulated to be shed into the blood circulation in prostate cancer patients, where they may cause an immune response. An antiprostasome antibody ELISA was developed and used to measure serum antibody titres in 81 males with prostate cancer or benign prostate hyperplasia. Patients with biopsy-verified prostate cancer had significantly higher antibody titres [p=0.019 for the dominant tumour expression (first Gleason parameter) and p=0.022 for the dominant and non-dominant form (first and second Gleason parameters)] than individuals with benign prostate hyperplasia or other benign prostate disorders. No correlation existed between antibody titres and PSA values. These results indicate that the antiprostasome antibody titre in serum may be a novel prostate cancer marker not replacing, but rather supplementing, PSA.