Although self-rated health is proposed for use in public health monitoring, previous reports on US levels and trends in self-rated health have shown ambiguous results. This study presents a comprehensive comparative analysis of responses to a common self-rated health question in 4 national surveys from 1971 to 2007: the National Health and Nutrition Examination Survey, Behavioral Risk Factor Surveillance System, National Health Interview Survey, and Current Population Survey. In addition to variation in the levels of self-rated health across surveys, striking discrepancies in time trends were observed. Whereas data from the Behavioral Risk Factor Surveillance System demonstrate that Americans were increasingly likely to report "fair" or "poor" health over the last decade, those from the Current Population Survey indicate the opposite trend. Subgroup analyses revealed that the greatest inconsistencies were among young respondents, Hispanics, and those without a high school education. Trends in "fair" or "poor" ratings were more inconsistent than trends in "excellent" ratings. The observed discrepancies elude simple explanations but suggest that self-rated health may be unsuitable for monitoring changes in population health over time. Analyses of socioeconomic disparities that use self-rated health may be particularly vulnerable to comparability problems, as inconsistencies are most pronounced among the lowest education group. More work is urgently needed on robust and comparable approaches to tracking population health.