In contrast to earlier reports, this study examined the relationship between plasma levels of factor VIII (FVIII) and von Willebrand factor (VWF) and ABO blood group and secretor status at the genetic level in 355 patients with venous thrombosis as well as in 236 controls. ABO glycosyl transferase alleles A(1) and B were more frequent in the thrombosis collective and alleles O(1), O(2) and A(2) were more frequent in the controls. A low-risk group for venous thrombosis of individuals with genotypes O(1)O(1), O(1)O(2) and O(1)A(2) (H-antigen rich) could be distinguished from a high-risk group with genotypes A(1)A(1), A(1)B, O(1)A(1) and O(1)B (H-antigen poor). In both the thrombosis and control groups, the H-antigen rich group showed significantly lower levels of FVIII coagulant activity (FVIII:C) and VWF antigen (VWF:Ag) than the H-antigen poor group. The frequency of the different secretor genotypes in the thrombosis group was not different from that in the control group. No significant differences of FVIII:C and VWF:Ag levels were seen between SeSe, Sese and sese individuals in the thrombosis and in the control group. Thus the risk of venous thrombosis is associated with the ABO blood group genotype but not with secretor status.