The dengue viruses (genus Flavivirus, family Flaviviridae) are mosquito borne and cause 100 million cases of dengue fever each year in most tropical and subtropical areas of the world. Increased global travel has been accompanied by an increased import not only of dengue but also of severe fevers of unknown origin to Sweden. Fifty-seven Swedish travelers to dengue epidemic areas, with clinical and serologically diagnosed dengue fever, were included in this study. To find fast and reliable methods to diagnose dengue in the early phase of the disease, patient acute-phase sera were investigated for the presence of dengue-specific immunoglobulin M (IgM) antibodies by enzyme-linked immunosorbent assay (ELISA) and also for dengue serotype (DEN-1 to DEN-4)-specific RNA by different PCR assays. The results showed that 15/20 (75%) of the samples collected 5 days or later post onset of disease, but only 5/37 (14%) of the samples collected on days 0 to 4, contained dengue-specific IgM. Of the samples collected on days 0 to 4 post onset, dengue RNAs of subtypes 1, 2, and 3 were detected by multiplex and/or by TaqMan PCR in 29/37 (78%); of these PCR-positive samples, 93% (27/29) were found IgM negative. By a combination of IgM ELISA and PCR assays, 84% (48/57) of the acute-phase samples were found to be positive. Our results demonstrated that detection of dengue viral RNA by reverse transcription-PCR and Taq-Man PCR is an excellent tool for the early diagnoses of dengue fever and that the IgM assay is a reliable complement for samples collected from day 5 post onset.