Thrombosis-inducing activity (TIA) was detected in the peripheral blood of patients with lung cancer who had undergone pulmonary resection. TIA was examined by intravenously injecting plasma from the patients into BALB/c mice. The plasma containing TIA induced multiple thromboses in the lung and caused the mice to die 3 to 30 minutes after injection. The subjects were 19 patients whose plasma contained no TIA before operation. TIA was detected in 5.3 percent (1/19) on the first postoperative day (POD), 47.4 percent (9/19) on the seventh POD, 47.1 percent (8/17) on the 14th POD, 26.7 percent (4/15) on the 21st POD, and 20 percent (2/10) on the 28th POD. Plasma fibrinogen levels and peripheral platelet counts increased postoperatively and reached a maximum on the seventh and 14th POD, respectively. Peripheral blood with TIA had a significant elevation of plasma fibrinogen levels and platelet counts as compared to that without TIA. These observations suggest that TIA is present in blood in a hypercoagulable state in patients after pulmonary resection. Since tumor necrosis factor and interleukin 1 are known to induce hypercoagulable states both in vitro and in vivo, we tried to determine whether it was possible to detect both cytokines in blood indicated as hypercoagulable state by the presence of TIA. They did not, however, reach detectable levels in the blood.