Most US medical records lack socioeconomic data, hindering studies of social gradients in health and ascertainment of whether study samples are representative of the general population. This study assessed the validity of a census-based approach in addressing these problems. Socioeconomic data from 1980 census tracts and block groups were matched to the 1985 membership records of a large prepaid health plan (n = 1.9 million), with the link provided by each individual's residential address. Among a subset of 14,420 Black and White members, comparisons were made of the association of individual, census tract, and census block-group socioeconomic measures with hypertension, height, smoking, and reproductive history. Census-level and individual-level socioeconomic measures were similarly associated with the selected health outcomes. Census data permitted assessing response bias due to missing individual-level socioeconomic data and also contextual effects involving the interaction of individual- and neighborhood-level socioeconomic traits. On the basis of block-group characteristics, health plan members generally were representative of the total population; persons in impoverished neighborhoods, however, were underrepresented. This census-based methodology offers a valid and useful approach to overcoming the absence of socioeconomic data in most US medical records.