Summary Research on dengue in the U. S. during World War II provided the following new information of special interest to military preventive medicine: 1. Proof of the existence of multiple immunological types of dengue. 2. The long persistence of immunity to homologous types of virus under conditions precluding reenforcement of immunity by subclinical reinfection. 3. The modifications of the clinical manifestations of the disease which result from reinfection with a heterologous type of virus at various periods after the primary attack. 4. The demonstration that in areas (e.g., New Guinea) where more than one imunological type of virus is present, fevers of unknown origin, clinically not recognizable as dengue, are actually caused by the dengue viruses. 5. The demonstration that type-specific immunity to dengue is associated with neutralizing antibodies for the virus, which can be used for diagnostic and epidemiologic survey purposes. 6. The propagation of dengue virus in mice with the resulting appearance of a mutant or variant strain which could be used for active immunization. In addition to the discoveries listed above, a great deal more was learned about the basic properties of the dengue viruses. Thus was dengue research brought from the field into the laboratory and further progress has been made possible by work on experimental animals instead of on human volunteers.