An increasing body of evidence suggests that diabetes mellitus constitutes a major health burden among the Mexican-American population. For example, county-wide death rates in Texas attributable to diabetes from 1970-1981 range from 2.5-52.0 diabetes deaths per 1000 total deaths with the highest rates generally occurring in counties whose populations are more than 75% Spanish ancestry. To assess the prevalence and morbidity of noninsulin-dependent diabetes mellitus among Mexican Americans, 14% of the Starr County, Texas, population (97% Mexican-American) was randomly sampled. The reference population, sampling strategy, and screening results are described. Age-specific prevalences of diabetes for males ranged from 0% in males aged 15-24 years to 17.6% in those above 75 years of age. Rates for females ranged from 0.4% in those aged 15-24 years to a high of 19.0% in the 55- to 64-year age group. In both sexes, the rates are relatively low for persons under age 45 with a sharp increase in those aged 45-54 years and high rates prevailing in the older age groups. Comparisons of the rates in Starr County to those of the general US population indicate a two- to fivefold greater risk in Starr County. In terms of impact on this community, these results imply that over 50% of individuals older than 35 years are directly affected by diabetes by virtue of their having the disease or by being a first-degree relative of a diabetic.