Metastasization may be associated with activation of haemostatic processes resulting in increased levels of circulating factor VIII-related antigen (FVIIIRAg) (von Willebrand factor antigen). To evaluate the relevancy of this in prostate cancer (PCa), the level of FVIIIRAg in the serum of patients with PCa, benign prostatic hypertrophy (BPH) and non-prostatic diseases was quantitated by a modified micro enzyme-linked immunosorbent assay. Significant (P less than 0.05) differences were noted between the level of FVIIIRAg in PCa and patients with BPH and other than prostatic disease. Noteworthy were elevated levels of FVIIIRAg in PCa patients with metastatic vs. localized disease. Consideration of the "unorthodox", but possibly more convenient use of routine serum specimens commonly available in the non-haematological laboratory vs. plasma for the quantitation of FVIIIRAg, in situations where an "absolute" level is not required, and of disseminated intravascular coagulation as contributory to the present observations, is given. Pending evaluation of a larger patient population these observations may be of prognostic value.