General self-rated health (SRH) is widely used to study trends and inequalities in population health. Recently, there has been an increased interest in understanding the measurement properties of SRH. This study evaluated for the first time the test-retest reliability of SRH among US adults. Analyses were based on a nationally representative sample of 9,235 adults interviewed in the 2005-2008 National Health and Nutrition Examination Survey (NHANES). Respondents reported SRH on 2 occasions (about 1 month apart). Kappa statistics, polyserial correlations, and agreement tabulations were used to assess reliability across population subgroups; regression models tested the association of sociodemographic factors and the stability of the rating. Nearly 40% of respondents changed their health rating between interviews, indicating moderate test-retest reliability of SRH. Reliability differed significantly by sociodemographic characteristics: Racial/ethnic minorities and adults with less education had lower reliability of SRH judgments. Health events between interviews did not influence consistency, but conditional on a rating change, they increased the odds of downgrading one's health. The results suggest that 1) there is a substantial amount of error in individuals' self-assessment of health and 2) reliability is worse for disadvantaged sociodemographic groups, potentially biasing estimates of health inequalities among US adults.