Six patients with hereditary anagioedema (HAE) undergoing 7 episodes of dental surgery received transfusions with fresh frozen plasma one day before surgery. Although the morbidity observed in these patients following similar procedures had been high, no significant complications of surgery were noted with this therapy. Thus, fresh frozen plasma infusion appears to provide a safe and effective method of prophylaxis in patients with HAE. Following infusion of fresh frozen plasma, serum levels of C4 esterase inhibitor (C1EI) rose transiently, and then fell to preinfusion levels within 1 to 12 days. In all but one patient the rise in C4 was greater than could be accounted for by the amount of C4 infused. In no patient did the level of C1EI or C4 rise to within the normal range. The data raise the question of the role of C1EI in the pathogenesis of angioedema in these patients.