An extensive epidemic of dengue type 1 infection with a high incidence of hemorrhagic manifestations occurred on the island of Viti Levu, Fiji beginning early in 1975. Previous dengue outbreaks in this population were such that in 1975 two types of dengue patients were observed, one group of patients experienced primary dengue type 1 infection, whereas another experienced their dengue type 1 infection approximately 4 years after a dengue type 2 infection. Clinical and laboratory findings for the two forms of infection were assessed in patients hospitalized for their disease, usually with hemorrhage. With the exception of virologic and serologic findings, no important differences between these two groups were noted with respect to incidence and nature of hemorrhage and other clinical and laboratory findings. Both types of infection sometimes were associated with thrombocytopenia and low serum levels of the C3 component of complement. Virus was more easily isolated from the primary infection patients and the magnitude of their viremia also was higher. No significant differences were noted in a comparison of the ease of virus isolation from serum or plasma.