In an attempt to explain the substantial inter-individual variability in half-life of infused factor VIII (FVIII) among haemophilia A patients, we studied the correlation of pre-infusion von Willebrand factor antigen (vWFAg) levels and the half-life of a recombinant-DNA-derived FVIII (r-VIII SQ. Pharmacia, Sweden). Intra-individual variation in half-life was small (range of variation 0-2.1 h) in 12 severely affected haemophiliacs who received two doses of FVIII concentrate. In contrast, inter-individual variation in half-life was large (range of half-lives 6-28.8 h). A strong correlation between half-life and pre-infusion vWFAg levels was present (r = 0.87, P = 0.0003). We conclude that the half-life of infused recombinant FVIII is related to pre-infusion vWFAg levels in severely affected haemophilia A patients.