Oral glucose tolerance tests with plasma glucose and insulin determinations were performed on 195 patients with impaired glucose tolerance. Patients were divided into three groups according to blood pressure levels: normal, below 140/90 mm Hg; diastolic hypertension, diastolic pressure above 90 mm Hg; and systolic hypertension, systolic pressure above 140 and diastolic pressure below 90 mm Hg. Sex, age, and glucose levels were similar among the groups. By contrast, serum insulin levels were significantly elevated for the patients with diastolic hypertension (p less than 0.01). This difference persisted after correction for body weight. These results suggest a causal relationship between the level of circulating insulin and diastolic blood pressure, and support the concept that hyperinsulinemia may be the common link in the clustering of hypertension, diabetes, and obesity.