OBJECTIVE To assess the effect of improved metabolic control with insulin therapy on blood pressure (BP) in elderly non-insulin-dependent diabetes mellitus (NIDDM) patients having secondary failure of oral antidiabetic drug therapy. METHODS Elderly NIDDM patients on maximal sulfonylurea treatment and showing poor metabolic control, but without symptoms of decompensation, were studied in a randomized prospective trial. One group was treated with insulin and the other was kept on oral treatment. Ambulatory BP measurements over 24 h were performed at the start and after 6 and 12 months. RESULTS Metabolic control in the insulin-treated, but not in the sulfonylurea-treated group, improved significantly. Body weight increased in the insulin-treated and decreased in the sulfonylurea-treated group. We found no changes in BP in the two treatment groups, whether studied during the 24-h period or during the daytime or night time when divided into longer (22.00 to 06.00 hours) or shorter (01.00 to 07.00 hours) time periods in the night. CONCLUSIONS No change in BP was noted in elderly diabetic patients when the metabolic control was improved by insulin treatment. Their weight gain may have counteracted the effect of the improved metabolic control on BP. Thus, elderly NIDDM patients seem not to have any benefits from insulin treatment as regards BP control.