Background: Prostate cancer (PC) is characterized by increased prothrombotic state due to enhanced tissue factor (TF) expression/procoagulant activity (PCA). We and others observed that GRP78, a molecular chaperone, is expressed on the surface of PC cells where it functions as a signaling receptor to promote cell proliferation and survival. Further, exposure of GRP78 on the surface of PC cells stimulates the production of anti-GRP78 autoantibodies in PC patients. We have reported that binding of these autoantibodies to cell surface GRP78 enhances TF PCA. We hypothesize that this autoantibody/cell surface GRP78 complex interaction increase PC progression and that disruption of this complex would represent a viable target for the treatment of advanced PC. Methods: Wild type, TF knockdown DU145 cells, and NOD/SCID mouse model system was used to investigate the effect of anti-GRP78 autoantibodies on tumour growth. Protein expression was determined using western blotting and qRT-PCR. TF activity was determined using the TF PCA continuous assay. Blood samples from patients diagnosed with PC were obtained from the Ontario Tumour Bank and St. Joseph’s Hamilton. Results: Pre-prostatectomy patients demonstrated high levels of anti-GRP78 autoantibodies (60-100µg/ml) vs. healthy individuals (5-10µg/ml). These titres were significantly reduced 24-weeks post prostatectomy. We show here that anti-GRP78 autoantibodies upregulate TF and its PCA through a mechanism involving depletion of ER Ca2+ stores. This very effect on ER Ca2+ stores also caused activation of the unfolded protein response, a pro-survival cellular pathway. Furthermore, these autoantibodies were shown to accelerate tumour growth in a NOD/SCID mouse model. Finally, heparin and low molecular weight heparin were shown to interfere with the binding of these antibodies to cancer cells and prevent PC cell activation. Conclusions: We have identified the function of an agent in patients' blood, anti-GRP78 autoantibodies, that correlate with PC stage in patients and increase TF PCA and promote PC progression in mice. The effect of this autoantibody can be reversed using heparin, thus, this acts as a new potential therapeutic target for PC.