"Cumulative naturalization rates through 2005 approached 60 percent for immigrants who had obtained legal permanent resident (LPR) status at least 20 years earlier. Naturalizations grew at a record pace between 2006 and 2008 with a total of 2.4 million immigrants becoming new citizens in the United States. This Office of Immigration Statistics Fact Sheet provides updated information on the cumulative naturalization rates and the timing of naturalization through 2008 of immigrants who obtained legal permanent resident status from 1973 through 1999. Data were obtained from matched LPR and naturalization records from DHS administrative systems. Cumulative naturalization rates through 2005 and 2008 and 10-year naturalization rates through 2008 were calculated for each LPR cohort. The data were restricted to immigrants who were 18 years of age and older when LPR status was obtained. The naturalization rates for all immigrant cohorts grew from 2005 through 2008, with the greatest increases occurring among the most recent cohorts. Among earlier cohorts, including immigrants who obtained LPR status from the early 1970s through mid 1980s, naturalization rates increased several percentage points-to as high as 63 percent-by 2008. More recent cohorts, with less time spent in LPR status, had lower cumulative naturalization rates."
United States Department of Homeland Security: http://www.dhs.gov/