Misperceptions about health conditions and behaviors may play a role in shaping health behaviors. Health messages frequently cite prevalence information in an effort to raise people's awareness of various health issues under the assumption that correcting misperceptions will increase healthy behavior. However, there is much to learn about the accuracy of estimates of the prevalence of prominent health conditions and behaviors among United States adults. We examined prevalence perceptions regarding a wide range of health conditions (obesity, diabetes, HIV infections, and HPV infections), health-risk behaviors (cigarette/e-cigarette use and binge drinking), and health-promotion behaviors (vegetable/fruit consumption, physical exercises, vaccination, and cancer screening) with a sample of U.S. adults stratified by race. We also examined perceptions of racial health disparities between white and black Americans. Respondents systematically overestimated the prevalence of health conditions and health-risk behaviors but underestimated the prevalence of health-promotion behaviors. Perceptions of racial disparities were comparable between white and black respondents. We end with a discussion of various implications related to misperceptions of prevalence estimates.