A circulating anticoagulant against factor XII was detected in a patient with smoldering leukemia. Despite severe associated thrombocytopenia the patient suffered two thromboembolic episodes, besides mucosal bleeding. The circulating anticoagulant was demonstrated not only in the plasma but also in the serum. Its activity was not affected by heating at 56° C for 30 min and it was not adsorbed by S04Ba or A1(OH)3. The circulating anticoagulant was not dializable and demonstrated to be an IgG. This is apparently the first reported association of smoldering leukemia and a circulating anticoagulant against factor XII