We report the case of a 25-year-old male Japanese homosexual with primary human immunodeficiency virus (HIV)-1 infection and early stage syphilis. Approximately 60 days after HIV exposure by sex with another man, the patient abruptly had high fever, after which he experienced a variety of severe, prolonged symptoms such as painful oral mucosa ulcerations, rash, lymphadenopathy, splenomegaly, and a 5.5-kg weight loss. Serum lactate dehydrogenase and liver biochemical test values were elevated. Antibodies to HIV by both enzyme-linked immunosorbent assay (ELISA) and Western blot (WB) test were negative at the time of symptom onset, but serum HIV-1 RNA level was 1 585 000 copies/ml. Antibody seroconversions were found on day 9 after the onset of symptoms by ELISA and on day 16 by WB test, suggesting primary HIV infection. Within 2 weeks of starting highly active antiretroviral therapy (HAART), all symptoms except lymphadenopathy were resolved, and the serum HIV-1 RNA level dramatically decreased to 5011 copies/ml, eventually becoming undetectable by the standard method. The patient has remained asymptomatic for the 18 months since symptom resolution after HAART, and HIV-1 RNA remains undetectable.