The relationship between diabetes and hypertension in 3456 residents of Rancho Bernardo, California, aged 50-79 years, surveyed in 1972-1974, was analyzed in depth in both the univariate mode and after adjustments for the potential confounding effects of age, obesity and diuretic medication. An association between diabetes and hypertension was present in both men and women at all ages, and the association was strongest for subjects having the best evidence for diabetes, i.e., both historical diabetes and fasting hyperglycemia. Adjustment for obesity reduced the association considerably, but a consistent association remained. diabetes and hypertension are linked only partially by obesity. Some of the excess risk of coronary heart disease in diabetes is probably due to hypertension.