The level of fibrin degradation products (fdp) as a marker of fibrin clot dissolution was studied prospectively in 51 patients with phlebographically identified deep vein thrombosis (DVT). For this purpose a highly sensitive fdp assay using an anti D neo monoclonal antibody (McAb) was used. At the onset of the hospitalization, in 47 (92%) of the 51 patients tested, the plasma fdp level performed was high, but does not reflect the size of the thrombus, demonstrating that spontaneous thrombus lysis varies from one patient to another. During 10 days of standard heparin or low molecular weight heparin treatment, two different patterns of fdp evolution could be identified in these patients, independent of the type of heparin used. The first was characterized by a gradual decrease in fdp level and a corresponding reduction in the thrombus size. The second pattern showed a persistence of high levels of fdp after 10 days of therapy although the phlebographic score reveals a poor or partial response indicating that fibrinolysis or the balance of thrombus formation/fibrinolysis did not insure total thrombus dissolution. The 4 patients whose initial plasma fdp levels were only slightly increased during the 10 days, seem to have poor thrombolysis, as was shown by the unmodified phlebographic score after 10 days of treatment. Consequently, we conclude that the investigation of plasma fdp levels with a highly sensitive assay should contribute to the evaluation of thrombus evolution in DVT.