Acute pyrexia of unknown origin (Acute PUO) was reported to affect approximately 200,000-400,000 patients each year reported by the national Annual Epidemiological Surveillance Report. The patients usually present with fever of less than two-week duration and non-specific symptoms such as malaise, myalgia, headache and loss of appetite. Its mortality rate is less than 0.02 percent. It would be interesting to find the etiologies and propose a management plan if the etiologies are discovered.This prospective epidemiologic study aimed to discover the etiologies of acute undifferentiated febrile illness in a tropical region like Thailand.Ten community-based hospitals were chosen as representatives in each part of Thailand to enroll patients into the study. Patients aged over two years old who presented with fever at the participating hospitals during year 1991-1993 were eligible for the study. Entry criteria of acute undifferentiated febrile illnesses (AUFI) included oral temperature over 38.3 degrees C within the last 24 hours, duration of fever ranging from 3-14 days, no specific single organ involvement by history taking and physical examination, normal or non-specific results of the following investigations: complete blood count, thick film for malaria, urinalysis and chest roentgenogram. The patients were hospitalized and a preset diagnostic protocol was performed. Other diagnostic procedures deemed necessary by attending physicians were perform. Patients were followed up within one month after hospital discharge.1,240 patients were enrolled but only 1,137 case records and results of the serological tests were available for analysis. Etiologies could be found in 471 cases (38.7%). Primary bacteremia was detected in 36 cases (3.2%). E. coli, streptococci, salmonella, Enterobacter spp. and S. aureus were the five most common blood isolates. Serological studies revealed positive results for scrub typhus (7.5%), influenza (6.0%), dengue fever (5.7%), murine typhus (5.3%), enteric fever (1.9%), chikunkunya infection (1.1%), leptospirosis (1.1%) and melioidosis (0.9%). Thirteen cases succumbed (1.1%) in this study.The etiologies in the majority (61.3%) of AUFI remained unknown. Rickettsial infection, influenza and dengue fever are the most common identifiable diseases in a tropical country like Thailand especially during the rainy season. A management guideline for diagnosis and treatment of the AUFI with emphasis on primary bacteremia and antimicrobial-treatable AUFI was proposed.