Data from the Boston Area Community Health Survey show that both undiagnosed signs and symptoms and diagnosed type 2 diabetes mellitus (T2DM) are patterned by socioeconomic status (SES). Such patterning is corroborated by National Health and Nutrition Examination Survey data for diagnosed T2DM. Complementary data from an experiment concerning clinical decision making show T2DM is patterned by race/ethnicity, following diagnosis by a physician. Undiagnosed signs and symptoms of T2DM in the community are patterned by SES (rather than race/ethnicity), but following diagnosis by primary care physicians they are patterned more by race/ethnicity (rather than by SES). Race/ethnicity and SES in the United States are almost totally confounded, such that measuring one is essentially also measuring the other. Physician patterning of T2DM by race/ethnicity, however, motivates the search for genetic and biophysiologic explanations and distracts attention from the more important contribution of SES circumstances to the prevalence of diabetes mellitus.