In Bangkok, Thailand, 49 of 127 patients with dengue hemorrhagic fever experienced shock. The concentration of nine complement proteins measured in serial serum samples decreased during shock with the exception of C9. C3 and C5 were reduced to 20 to 40 per cent of normal in severe cases. Decrease of plasma fibrinogen, appearance of fibrinogen split products and thrombocytopenia indicated occurence of intravascular coagulation. Metabolic studies of C3 and C1q, performed on 24 patients, indicated a markedly enhanced fractional catabolic rate especially during shock. These results support the concept that activation of complement can constitute a major factor in the pathogenesis of dengue hemorrhagic shock. (N Engl J Med 289:996–1000, 1973)