Acute undifferentiated fever (AUF) is a temporary febrile illness accompanied by non-specific symptoms. Previous studies in Asia have primarily focused on delineating specific agent(s) causing AUF. None were designed to assess the frequency of undiagnosed cases. This study aimed to review the case definition, laboratory investigations, etiologies and proportion of undiagnosed episodes of AUF. We reviewed nine studies, each employing different case definitions and diagnostic tools. Malaria, dengue, leptospirosis and rickettsial illnesses were frequently identified as the etiologies of AUF and the frequencies of undiagnosed cases ranged from 8% to 80%. An international consensus definition is required to compare the occurrence of AUF in different geographical sites, particularly if this condition were to be used as an indicator for the emergence or re-emergence of infectious agents. Use of general diagnostic tools for infectious diseases might reduce the proportion of undiagnosed AUF cases.