The most recent data on ancestry come from the American Community Survey (ACS) and can be found on the American FactFinder (AFF).
In the fall of each year, data become available for the prior year. For example, 2010 ACS data became available in the fall of 2011.
The tables that show ancestry on AFF are:
There are also collapsed versions of these tables (C04001 through C04007) which show fewer categories but for smaller geographic areas.
Data on ancestry are available from ACS on AFF for the years 2000 and beyond.
Census 2000Data are available on American FactFinder (AFF) from Census 2000.
1990 Census (and 1980 Census) collected ancestry data the same way they are today.
1980 Census (and 1990 Census) collected ancestry data the same way they are today.