Abstract The biological half-life (T B) of 125I-fibrinogen and detection of deep venous thrombosis (DVT) have been studied simultaneously in 153 patients after major surgery. Multiple regression analyses of the influence from various clinical factors on the shortened T B indicated operation time as the most important factor. Malignancy not removed at operation, postoperative infections and DVT were also significant, but less important. The frequency of DVT was significantly increased in the postoperative patients receiving blood transfusions. It is concluded that the T B of 125I-fibrinogen is not useful in the diagnosis of DVT in the postoperative period, when other clinical factors with significant influence on the T B are present. The possibility of using the T B for early diagnosis of disseminated intravascular coagulation (DIC) is discussed.