Surface-mediated reactions of clotting were compared in 21 black children with homozygous sickle cell disease, 12 age-matched controls, and 15 adults. Both the coagulant and antigen titers of Hageman factor (factor XII) were decreased in asymptomatic patients compared with those in the control groups. These findings were associated with slight but significant reductions in the plasma titers of prekallikrein and high molecular weight kininogen. A further decrease from the initially low titers of these contact factors was observed during vaso-occlusive crises. Additionally, we observed a disparate relationship between Hageman factor coagulant activity and its antigen titers. These data provide evidence for reduction of the contact factors in patients with homozygous sickle cell disease.