The relationship between diabetes and parental diabetes status and obesity in Oklahoma Indians was studied. Data from 2095 adult Oklahoma Indians (1085 type II diabetic subjects and 1010 nondiabetic subjects) through a complete physical examination and personal interview showed a strong association between diabetes and parental diabetes status. Frequency of diabetes among siblings was significantly higher in families with affected parents than those without diabetic parents. No significant difference was found between families with one diabetic parent and those with two diabetic parents. The diabetic individuals were more obese than the nondiabetic individuals at age 18 and at interview. Obesity was defined as percent body mass index >120. After adjusting for possible age and sex effects, the risk of diabetes for the obese was estimated as almost twice that for the nonobese.