We report an incidence of thrombosis of 17.6% in 159 patients treated with a five-drug chemotherapy regimen (cyclophosphamide, methotrexate, 5-fluorouracil, vincristine, and prednisone) for Stage IV breast carcinoma. Chi-squared analysis of risk factors for thrombosis (ambulatory status, obesity, family history, smoking, diabetes mellitus, hypertension, liver dysfunction, thrombocytosis, and previous endocrine therapy) showed no difference between the patients who had a thromboembolic event and those who did not. Statistical analysis revealed that a significantly higher incidence of thrombosis occurred during the chemotherapy regimen than when off this regimen (P less than 0.05). Detailed coagulation studies done prospectively on 10 patients receiving the five-drug chemotherapy regimen compared with 10 control patients showed a significantly elevated Factor VIII antigen:activity ratio in the group receiving the chemotherapy regimen compared with the control group and normals. These results implicate the chemotherapeutic regimen in the pathogenesis of the increased incidence of thrombosis. The pathophysiology of thrombosis in settings such as this awaits better in vitro tests defining the "hypercoagulable state."