Of the many tropical diseases of potential hazard to American troops in South Vietnam, a significant number are characterized by the non-specific triad of chills, high fever, and headache, with or without a variety of associated symptoms. In an attempt to define these "fevers of unknown origin", 110 patients presenting in this fashion in whom a diagnosis could not be made within 24 hours of admission were studied. A standard clinical protocol designed to monitor symptoms, signs and basic laboratory data was instituted in each case. Appropriate serologic, virologic and bacteriologic studies were performed for the following diseases: dengue, chikungunya, Japanese encephalitis, infectious mononucleosis, scrub typhus, typhus fever group, leptospirosis, brucellosis, melioidosis, salmonellosis, shigellosis, and marlaria. The major diseases uncovered by this study can be divided into 6 major groups: arbovirus diseases (Dengue, Chikungunya), scrub typhus, malaria, enteric infections, specific miscellaneous diseases and febrile diseases of undetermined etiology. Seventy-three percent of the cases could be definitely diagnosed. Upon analysis of the clinical protocols of these patients, a number of significant diagnostic features, which have not previously received enough emphasis, became evident, and were clearly of value in early diagnosis. The differential diagnosis and epidemiologic significance of these tropical febrile diseases is discussed.