"This Office of Immigration Statistics Fact Sheet provides information on trends in the population without health insurance coverage by nativity and citizenship status. Data were obtained from the Annual Social and Economic (ASEC) Supplement to the Current Population Survey (CPS) of the U.S. Census Bureau. In the ASEC Supplement, questions regarding health insurance coverage ask respondents if they were covered by specific types of health insurance at any time during the previous year (i.e., 'At any time in 2007, were you covered by Medicare?'). Estimates of the uninsured population presented in this report therefore refer to the number of individuals not covered by health insurance during the entire year."
Department of Homeland Security: http://www.dhs.gov/