In 29 patients with dengue hemorrhagic fever (DHF), 12 with grade 2 and 17 with grades 3 and 4, fibrinogen metabolism was studied by using 125I-fibrinogen; 11 of these patients were studied during shock. Hemostatic studies were also performed to search for evidence of disseminated intravascular coagulation (DIC). Increased intravascular coagulation, as judged by rapid T1/2 of 125I-fibrinogen, as well as evidence of DIC by hemostatic patients with DHF grade 2 had rapid T1/2 and only 17% had DIC. Of the 11 patients studied during shock, 91% had both rapid T1/2 and evidence of DIC, whereas 63% of the 18 patients without shock had rapid T1/2 and only 11% of this latter group had DIC. A correlation between the increased fibrinogen consumption as judged by rapid T1/2 of 125I-fibrinogen, DIC, clinical severity, and shock was demonstrated. The role of DIC in the pathogenesis of DHF is discussed, and heparin is suggested for patients with prolonged shock and severe acidosis when DIC becomes clinically apparent.