Fibrinopeptide A (FPA) levels have been followed sequentially in a three-year study of 50 patients with advanced carcinoma. Evidence for activation of blood coagulation was found in 26 of 43 subjects (60%) at the time of entry into the study. Serial FPA determinations revealed an upward trend which paralleled the progression of clinical disease. Persistent elevation of the FPA level suggested treatment failure and a poor prognosis. Anticoagulation with sodium warfarin significantly reduced the FPA level in subjects with cancer. Short-term anticoagulation with heparin decreased FPA levels in two patients with thromboembolic disease but failed to reduce FPA to the normal range in any of the three patients with cancer so tested. These data suggest that most patients with advanced cancer have evidence for activation of blood coagulation and suggest that serial FPA determinations may be useful in following tumor progression or response to therapy in patients with cancer.