A clinical and virologic investigation of the amount and nature of disease caused by infection with dengue and chikungunya viruses in 323 outpatients in Bangkok Children's Hospital was done during 1962–1964. For 317 patients, attempts to isolate enterovirus and arbovirus and arbovirus serologic studies were done. Of these patients, 26 (8.0%) had chikungunya and 94 (29.1%) had dengue infection. Of the latter, 66% had secondary type of group B antibody responses. A positive tourniquet test, cool extremities, and palpable liver were seen with equal frequency in patients with either primary or secondary dengue infections. Thrombocytopenia was observed only during secondary dengue infection. Males predominated in all etiologic categories except secondary dengue infections. Nearly 50% of patients with chikungunya or with primary or secondary dengue infections had a clinical diagnosis of “upper-respiratory illness.” We concluded that 1) capillary damage and moderate liver enlargement may be a part of the “normal” pathologic response to primary dengue infection in children; 2) during secondary dengue infections there may be a clinical continuum from attenuated illness to “normal” dengue to disease characterized by severe capillary damage and abnormalities in the blood-clotting mechanism (hemorrhagic fever); 3) secondary dengue infections are more severe in females than males; and 4) chikungunya and dengue viruses are a cause of outpatient “upper-respiratory illness” at epidemic proportions.