Fourteen tests of hæmostasis and fibrinolysis were done before and after major elective surgery in 73 patients. The patients were also studied with 125I-fibrinogen scanning, and 27 developed scan evidence of postoperative leg-vein thrombosis. Patients with thrombosis had a significantly shorter partial thromboplastin-time (P.T.T.) before surgery (mean 35.9 seconds) than patients without thrombosis (mean 38·8 seconds). They also had a significantly shorter P.T.T., higher antiplasmin level, and higher serum fibrin/fibrinogen degradation product level on the first postoperative day. The P.T.T, was the only test to show any difference between the two groups after this. These results suggest that blood tests done before and after surgery can detect differences beween patients with and without postoperative leg-vein thrombosis.