The in vivo recovery of factor VIII has been estimated on 84 occasions in 53 severely affected adolescent haemophiliacs. There was wide individual variation in recovery, which was not affected by differences in the administered dose. Recovery increased steadily with increasing surface area, and it was only over a surface area of 1.7 m2 that a recovery of 2% of factor VIII per unit per kg became the norm. It is suggested that the only safe assumption to make below that surface area in an in vivo recovery of 1.5%.