To evaluate the effect of major improvement in glycemic control on cardiovascular function tests in patients with insulin-dependent diabetes mellitus (IDDM).Eight men with chronic, poorly controlled IDDM (serum fructosamine >300 mmol/L and mean fasting blood glucose >200 mg/dL), who were 24 +/- 6 years of age and had a mean duration of diabetes of 8 +/- 1 years, were studied. No patient had microalbuminuria, was taking medication other than insulin, or had evidence of heart disease. Patients underwent baseline continuous ambulatory blood pressure monitoring, echocardiographic studies, and cardiovascular autonomic function tests, which were repeated after 4 weeks of an intensified insulin treatment program (IITP), during which they reached and maintained blood glucose concentrations and serum fructosamine levels in near-normal, nondiabetic ranges.Substantial changes in glycemic control had no significant influence on results of ambulatory blood pressure monitoring, cardiovascular autonomic function tests, and echocardiographic studies.Major improvement in glycemic control during a 1-month period in patients with IDDM had no significant influence on cardiovascular function tests. We cannot exclude the possibility that, after a longer duration of an IITP or in patients with clinically evident heart disease or evidence of major complications of diabetes, different responses might be observed.