Pentosan polysulphate is an heparin analogue which acts via an antithrombin III (AT III) independent pathway. We compared the effect of this drug to that of heparin and AT III infusions in AT III deficient patients. Four patients with AT III congenital deficiency received on three different occasions: (i) an infusion of human AT III concentrate (20 U/kg or 40 U/kg), (ii) an intramuscular injection of pentosan polysulphate (2 mg/kg), (iii) a subcutaneous calcium heparin injection (100 U/kg). AT III infusion inhibits the excessive thrombin generation (46% of inhibition) observed in the plasma of AT III deficient patients during at least 12 hours, but does not modify the factor Xa formation. On the contrary, pentosan polysulphate has a marked effect on both thrombin (62% of inhibition) and factor Xa generation (57% of inhibition) still present 8 hours after injection. Heparin injection has the same effect, more prolonged, as pentosan polysulphate on thrombin generation but is not so effective on impairing factor Xa generation (27% of inhibition). The marked effect of pentosan polysulphate on thrombin and factor Xa generation in these patients is due to its AT III independent mechanism of action.