It has been suggested that the clinical course of severe hemophilia A can be influenced by coinheritance of molecular defects that alone cause increased risk of venous thromboembolism at an early age. We have investigated 31 Bulgarian patients suffering from severe hemophilia A and B below 1%, as determined by factor VIII and factor IX activity levels, respectively. Three hemophilia A patients (10%) had a surprisingly mild presentation. The heterozygous factor V Leiden mutation (G1691A) was found in one patient who did not display amelioration in the clinical phenotype. Thus, the present study suggests that in the Bulgarian hemophiliac population, a milder clinical course cannot be attributed to coinheritance of the prothrombotic risk factor factor V Leiden, or the proportion of such cases in which the prothrombotic defect moderates the underlying coagulation defect, is quite small.

Key words: hemophilia, prothrombotic risk factor, genotype.