In vitro investigations have demonstrated a high F VIII:Rcof potency and a high F VIII:Rcof/F VIII R:Ag ratio of two heat-treated F VIII concentrates. We therefore studied the in vivo effectiveness of these preparations (F VIII HSR, Behringwerke Marburg and F VIII HTR, Travenol) in five patients with von Willebrand's disease (vWd). In the steady state in vivo recoveries of F VIII:Rcof ranged from 73-153% after transfusion of F VIII HSR and from 11.5-17% after F VIII HTR respectively. The gain of F VIII-complex after F VIII HS was comparable to cryopecipitate (KryobulinR SP, Immuno AG Wien). All three products shortened the bleeding-time. Three of our five patients underwent surgery (Billroth I, papillotomy, laparatomy, open heart surgery) under F VIII HS cover without bleeding complications. The dose applied ranged from 20 to 40 U/kg at 8 or 12 h intervals for a period of approx. 14 days. Serum-transaminase elevations were observed in two of four patients after F VIII HT treatment. Although the risk of hepatitis of heat-treated F VIII concentrates remains to be determined, these products proved to be effective in vWd. The major advantages of these preparations are stability, rapid solubility, a low content of contaminating proteins, and a rapid, general availability.