Summary. When 125I‐fibrinogen scanning is used for the early detection of venous thrombosis, comparison with venography has shown a small percentage of abnormal scans associated with normal venograms. The mechanism of this discrepancy was investigated in an experimental model. Venous thrombosis was produced in 56 dogs using stasis and local thrombin. The data obtained from scanning and venography were validated by direct vein examination. 125I scanning agreed in 96% with venography when thrombus was present, but when thrombus lysed or embolized scans continued to read positive even though venograms were negative. Deliberate embolization of thrombi showed that these false positive scans continued for up to 48 hr owing to activity persisting in the vein wall. This was shown on histology to be related to fibrin deposition in the sub‐endothelial and adventitial layers with inflammatory changes.