During 1974, 114 patients with dengue hemorrhagic fever were studied at the Bangkok Children's Hospital. Over 40% of the patients had dengue shock syndrome. Five fatal cases were included in the study. Primary dengue infections were identified by absent or low titered antibodies in acute sera and the sequential development of IgM antibodies followed by IgG antibodies during convalescence. Three patients, aged 4, 8, and 12 years, had primary dengue infections with shock. Although no convalescent sera could be tested two other patients, aged 7 and 12 years, with fatal disease also appeared to have primary infections. At the time of shock patients with primary infections had subnormal concentrations of complement factor 3. The data show that in older children dengue shock syndrome associated with complement depression can occur during primary as well as secondary infections.