Dengue fever (DF) is a relatively common infection in travelers, with about 100 cases being reported annually in Japan, and this number is increasing. We herein describe two patients who developed a fever after returning to Japan from Southeast Asia and who were serologically diagnosed with DF. Patient 1 was a 19-year-old man who spent 6 days in Thailand and developed diarrhea and a fever after returning to Japan. Virological studies showed dengue virus (DV) serotype 3 by reverse transcriptase PCR (RT-PCR), and anti-DV IgM and IgG antibodies were both positive by enzyme-linked immunosorbent assay (ELISA). Patient 2 was a 43-year-old man who spent time in various Asian countries and developed a fever and arthralgia after returning to Japan. Virological studies showed DV serotype 2 by RT-PCR, and anti-DV IgM and IgG antibodies were both positive by ELISA. DF and other febrile diseases, including Chikungunya fever, should be strongly suspected in patients who develop fever after returning to Japan from other Asian countries, irrespective of whether patients remember being bitten by mosquitoes.