This study was designated to describe clinical and biological features of patients with a suspected diagnosis of dengue fever/dengue hemorrhagic fever during an outbreak in Central Vietnam. One hundred and twenty-five consecutive patients hospitalized at Khanh Hoa and Binh Thuan Provincial hospitals between November 2001 and January 2002 with a diagnosis of suspected dengue infection were included in the present study. Viruses were isolated in C6/36 and VERO E6 cell cultures or detected by RT-PCR. A hemagglutination-inhibition test (HI) was done on each paired sera using dengue antigens type 1-4, Japanese encephalitis (JE) virus antigen, Chickungunya virus antigen and Sindbis virus antigen. Anti-dengue and anti-JE virus IgM were measured by a capture enzyme-linked immunosorbent assay (MAC-ELISA). Anti-dengue and anti-JE virus IgG were measured by an ELISA test. Dengue viruses were isolated in cell culture and/or detected by RT-PCR in 20.8% of blood samples. DEN-4 and DEN-2 serotypes were found in 18.4% and 2.4% of the patients, respectively. A total of 86.4% of individuals had a diagnosis of acute dengue fever by using the HI test and/or dengue virus-specific IgM capture-ELISA and/or virus isolation and/or RT-PCR. The prevalence of primary and secondary acute dengue infection was 4% and 78.4%, respectively. Anti-dengue IgG ELISA test was positive in 88.8% of the patients. In 5 cases (4%), Japanese encephalitis virus infection was positive by serology but the cell culture was negative. No Chickungunya virus or Sindbis virus infection was detected by the HI test. In patients with acute dengue virus infection, the most common presenting symptom was headache, followed by conjunctivitis, petechial rash, muscle and joint pain, nausea and abdominal pain. Four percent of hospitalized patients were classified as dengue hemorrhagic fever. The clinical presentation and blood cell counts were similar between patients hospitalized with acute dengue fever and patients with other febrile illnesses.