One new case and 29 reported cases of hereditary prekallikrein (Fletcher factor) deficiency are reviewed. Abnormalities in the coagulation, fibrinolytic, complement, and kinin systems are described. These cases are discovered incidentally by prolonged partial thromboplastin times (PTTs) which correct with extended incubation in the presence of a contact activator. Prekallikrein levels are less than two percent of normal levels. In general, the remainder of the coagulation profile is normal, and no bleeding diathesis is present. Most patients are black and the incidence of consanguinity is increased. The disease is transmitted in an autosomal recessive manner. Acquired Fletcher factor disease is a moderate prekallikrein deficiency present in many common disease states. Its clinical significance is largely unknown. Both acquired and hereditary forms may rarely predispose to thrombotic phenomena.