"This report presents estimates of the legal permanent resident (LPR) population living in the United States on January 1, 2011. The LPR population includes persons granted lawful permanent residence i.e., 'green card' recipients, who have not yet become U.S. citizens. The estimates are shown for the total LPR population and the LPR population eligible to apply to naturalize by country of birth, state of residence, and the year LPR status was obtained. Data for the estimates were obtained primarily from administrative records of U.S. Citizenship and Immigration Services (USCIS) of the Department of Homeland Security (DHS). […] In summary, an estimated 13.1 million LPRs lived in the United States on January 1, 2011. The majority (59 percent) obtained LPR status in 2000 or later. Of the total 13.1 million LPRs, an estimated 8.5 million were eligible to naturalize."
U.S. Dept. of Homeland Security: http://www.dhs.gov/