Virological studies were carried out during an epidemic of dengue hemorrhagic fever in Central Java, Indonesia in 1976. Dengue virus was isolated from the acute sera of 45 of 69 patients (65%). The isolation rate was higher in primary than secondary cases. Dengue 3 was the predominant serotype being transmitted (27 isolates), but both dengue 1 (8 isolates) and dengue 4 (10 isolates) were also being transmitted. A composite picture of magnitude and duration of viremia showed that many patients were circulating over 10 (8) MID50 per milliliter dengue 3 virus for the first 3 days of illness and that viremia persisted for 5-6 days in some persons. If all shock cases were considered, there was no relationship between dengue serotype and severity of disease. All three confirmed fatal cases, however, were associated with dengue type 3 infections.