This report compares adult mental health prevalence estimates generated from the 2009 National Survey on Drug Use and Health (NSDUH) with estimates of similar measures generated from other national data sources. It also describes the methodologies of the different data sources and discusses the differences in survey design and estimation that may contribute to differences among these estimates. The other data systems discussed include the 2001 to 2003 National Comorbidity Survey Replication (NCS-R), 2001 to 2002 National Epidemiologic Survey on Alcohol and Related Conditions (NESARC), 2007 Behavioral Risk Factor Surveillance System (BRFSS), 2008 National Health Interview Survey (NHIS), 2008 Medical Expenditure Panel Survey (MEPS), and 2008 Uniform Reporting System (URS). Mental health indicators compared include past year serious mental illness (SMI), past year any mental illness (AMI), past month serious psychological distress (SPD), past year major depressive episode (MDE), and past year suicidality. Although methodological differences between surveys make interpretation difficult, there were multiple differences in estimates of mental health indicators between the data sources. Estimates of past year SMI and AMI from NSDUH were significantly lower than estimates of these indicators from NCS-R. The prevalence rates of past month SPD from NSDUH were significantly higher than the estimate from BRFSS or NHIS and similar to the estimate from MEPS. Estimates of past year MDE were lower in NSDUH than in NCS-R or NESARC. The NSDUH estimate of past year suicide ideation was higher than the estimate from NCS-R, but there were no differences between these data sources in respondents having suicide plans or making suicide attempts in the past year. Conclusions regarding which estimates of these mental health indicators are "correct" are difficult due to substantial methodological differences across the surveys (e.g., year of data collection, sampling design, mode of data collection, specific measures used, and operational definitions). However, precise agreement among the surveys is not expected, and this lack of agreement does not reduce the importance of these studies in providing a comprehensive picture of mental health in the United States.