A detailed, longitudinal study was undertaken to investigate the immunological and virological features of an individual with hemophilia infected with human immunodeficiency virus type-1 (HIV-1) for 10 years without disease. Methods applied to serial samples of peripheral blood included Western blot analysis, neutralizing antibody assays, antibody-dependent cell-mediated cytotoxicity (ADCC) titration, HIV-1 specific cytotoxic T lymphocyte (CTL) assays, viral cultures, and PCR with sequence analysis of viral regulatory genes. Strong antibody responses against HIV-1 antigens as measured by Western blot and ADCC assays have persisted throughout infection. Repeated attempts to isolate HIV-1 using sensitive culture techniques and to demonstrate viremia with standard PCR methods have failed. Using the "booster" PCR technique, a period of viremia in peripheral blood mononuclear cells was demonstrated. Concurrent with detection of circulating virus, titers of neutralizing antibodies and circulating HIV-1-specific CTLs became measurable. Sequencing studies of a portion of the viral genome showed no significant abnormalities of the regulatory genes. In this individual, the combination of low viral load in the peripheral blood and a strong, responsive immune system is associated with long-term, disease-free coexistence with HIV-1 infection.