Summary Acute- and convalescent-phase sera from a series of Bangkok Hospital patients of 1960 with clinical diagnoses of hemorrhagic fever, dengue, fever of unknown origin (FUO) and other diseases were tested rather extensively for arbovirus isolations and serological responses. Five dengue virus isolations were made from acute-phase sera, one of which was typed in the type 1–6 complex, another in the 2–5 complex, two as type 4, and one as yet untypable. Several probable latent mouse viruses were also encountered. A number of cases of hemorrhagic fever, dengue or FUO was serologically confirmed with demonstrated antibody rises by complement fixation, neutralization, or hemagglutination inhibition as group-B arbovirus infections, probably due to dengue viruses. None was confirmed as a chikungunya virus infection, although several gave indication of recent infection. Antibody rises to group B fell in most cases into either of two patterns, group-B primary infections or those with previous group-B experience. These two types of responses were quite different. Further evidence was found to support calling TH-36 and TH-Sman, dengue prototypes previously isolated in Bangkok, dengue types 5 and 6 respectively, although they are closely related to types 2 and 1. Human primary group-B convalescent-phase sera, but not animal sera, usually permitted clear differentiation. Difficulties of dengue virus isolation, adaptation and typing, and limitations of serological studies are described. No evidence was found for the activity of viruses in the Bunyamwera group, California group, or Sindbis and Eastern equine viruses in group A. Japanese B, Murray Valley and Langat viruses in group B frequently gave positive reactions with sera from the dengue virus infected patients.