In order to elucidate the usefulness of various tests in the early course of dengue infection, in terms of diagnosis and correlation with clinical severity, blood specimens were collected every 48 hours on 3 occasions from patients with clinical suspicion of dengue infection with fever for less than 4 days. Viral isolation was attempted by mosquito inoculation (MI), tissue culture inoculation (TC), and reverse transcriptase polymerase chain reaction (RT-PCR). Antibodies were detected by hemagglutination inhibition test (HI), an in-house-ELISA (IH-ELISA), and an ELISA by MRL diagnostics Clinical data were collected from the time of enrollment to complete recovery. Of the 40 patients enrolled, 31 were diagnosed as dengue infection and confirmed by either serology or viral isolation. Of these, 12 had primary infection and 19 had secondary infection. Dengue fever occurred in 9 cases. Dengue viruses were isolated from 28 out of 31 patients, and dengue hemorrhagic fever was diagnosed in 22 patients. Viral serotypes identified by viral isolation, and RT-PCR were concordant: DEN1 was isolated in 8, DEN2 in 13, DEN3 in 5, and DEN4 in 2 patients. Viral isolation yielded positive results on blood collected before the 5th day of fever. MI was more sensitive than TC. RT-PCR was less sensitive than viral isolation during the early days of fever, but became more sensitive after the 5th day of fever. RT-PCR was able to detect virus up to day 7-8 of fever, even after defervescence, and in the presence of antibody. During the febrile stage, serological diagnosis on blood samples taken 48 hours apart was carried out by HI, IH-ELISA, and MRL-ELISA, facilitating diagnosis in 3 (10%), 21 (67%), and 27 (87%) of patients, respectively. All of the patients with secondary infection were diagnosed by MRL-ELISA before defervescence. By the 8th day of fever, a serological diagnosis aided to diagnose in 9 (29%), 29 (93%), and 31 (100%) of patients by HI, IH-ELISA, and MRL-ELISA, respectively.