We have carried out a 5 year follow-up study of a group of 41 originally normotensive (BP less than 160/95 mmHg) newly diagnosed Type 2 (non-insulin-dependent) diabetic patients (26 men, 15 women) and 86 non-diabetic subjects (39 men, 47 women) to assess the predictive value of serum insulin levels with regard to the development of hypertension. Hypertension (BP greater than 160/95 mmHg and/or drug treatment) developed in 14% of diabetic patients and 10% of non-diabetic subjects (NS). The baseline postglucose insulin levels tended to be higher in those diabetic and non-diabetic subjects who developed hypertension during the 5 year follow-up than in those who remained normotensive, and in non-diabetic subjects the differences were statistically significant after adjustment for age, sex and body mass index for the baseline 1 hour serum insulin (104 +/- 18 vs. 68 +/- 5 mU/l; P less than 0.05) and area under the insulin curve (138 +/- 34 vs. 85 +/- 8 mU/l.h, P less than 0.05). Both diabetic and non-diabetic subjects who developed hypertension showed elevated total- and VLDL-triglycerides at baseline compared with those subjects who remained normotensive during the follow-up. In conclusion, the results support the hypothesis that hyperinsulinaemia or insulin resistance may play a role in the pathogenesis of hypertension.