Accurate detection of active deep venous thrombosis with the fibrinogen uptake test (FUT) requires adherence to rigid criteria for interpretation of results of the tests. In a series of 56 patients, only detection of serial increase of per cent uptake at the same position correlated with presence of active venous thrombosis in all cases. Other investigated criteria, which were based on the analysis of single FUT results, showed a 21% incidence of false positive results. Accordingly, active venous thrombosis investigation by FUT requires performance of serial tests and analysis of the curve of uptake at each position to detect serial increasing per cent uptake.