To examine trends in HIV prevalence in the US household population, serum or urine samples from 2 National Health and Nutrition Examinations Surveys (NHANES) (1988-1994 and 1999-2002), were tested for HIV antibody. In the 1999 to 2002 survey, data on risk behaviors, CD4 T lymphocytes, and antiretroviral therapy (ART) were also available. In the 1988 to 1994 survey, there were 59 positive individuals of 11,203 tested. In NHANES 1999 to 2002, there were 32 positive individuals of 5926 tested. The prevalence of HIV infection among those aged 18 to 39 years in NHANES 1988 to 1994 was 0.38% (95% confidence interval [CI]: 0.22-0.68) as compared with 0.37% (95% CI: 0.17 to 0.80) in 1999 to 2002. Prevalence did not change significantly between surveys in any race and/or ethnic or gender group among 18- to 39-year-old participants. HIV prevalence was 3.58% (95% CI: 1.88 to 6.71) among non-Hispanic blacks in the 40- to 49-year-old age group in 1999 to 2002, but the age range available in NHANES 1988 to 1994 was 18 to 59 years and does not allow direct comparison of prevalence. Cocaine use and the presence of herpes simplex virus-2 antibody were the only significant risk factors for HIV infection for non-Hispanic blacks. Fifty-eight percent of infected individuals not reporting ART had CD4 T-lymphocyte counts <200 cells/mm3 compared with 18.2% on therapy and 12.5% of participants newly informed of their HIV status.