Up to 40% of patients with mild haemophilia A have a discrepancy whereby factor VIII (FVIII) measurements by a two-stage chromogenic assay (FVIII:C(CH)) are disproportionately reduced compared with the FVIII one-stage clotting value (FVIII:C). Which assay best reflects the coagulation potential and clinical phenotype in this patient group is of clinical significance, yet remains unclear. We have assessed the global coagulant ability of haemophilia patients with FVIII assay discrepancy using calibrated automated thrombography (CAT). A total of 18 patients with mutations Arg531His/Cys or Arg698Trp causing FVIII discrepancy were investigated, together with 12 haemophilia patients with concordant FVIII values and 15 normal controls. Factor VIII levels in all patients and controls were measured using both one-stage clotting assay and two-stage chromogenic assay. Thrombin generation was assessed in platelet-poor plasma by CAT using a low tissue factor concentration (1 pm). FVIII:C(CH) values were below normal in all patients, and in the discrepant group were between 1.5- and 8-fold lower than FVIII:C values. CAT parameters were affected in all haemophilia patients. The endogenous thrombin potential (ETP) was reduced to 58-67% of the mean normal value (1301 nm min(-1)), whereas peak thrombin was further reduced to 27-30% of the mean normal value (178 nm) in both discrepant and concordant patient groups. Analysis of the discrepant patient group showed the most significant correlation between the one-stage FVIII:C assay and ETP (r(2) = 0.44) and peak thrombin parameters (r(2) = 0.27).