"This report describes naturalization trends for immigrants who obtained lawful permanent resident (LPR) status between fiscal year (FY) 1973 and FY 2008. Naturalization is the process by which U.S. citizenship is granted to a foreign national who fulfills the requirements established by Congress in the Immigration and Nationality Act. The yearly number of naturalizations depends both on operational factors, such as application processing times, and other factors such as the number of LPRs eligible to naturalize at any given time, as well as their motivation to apply and ability to pay the associated fees. This fact sheet uses data from U.S. Citizenship and Immigration Services (USCIS) naturalization records4 to provide information on the rates of naturalizations by immigrant cohorts, regions of birth, selected countries of birth, and classes of admission."
U.S. Citizenship and Immigration Services: http://www.uscis.gov/