A 21 year old man with mild hemophilia A (factor VIII levels of 6.9) and 4.5% ) developed a factor VIII inhibitor of 12.5 units/ml after receiving concentrated human factor VIII for a surgical procedure. The inhibitor was undetectable six weeks after it appeared. Only four previous cases of inhibitors in mild hemophiliacs have been reported and the low titers and transient nature of the inhibitors in these cases make them unlike the inhibitors that occur in severe hemophiliacs.