Left ventricular (LV) diastolic dysfunction is a main feature of diabetic heart disease. The aim of this prospective study was to evaluate the influence of glycemic control on diastolic function in type 1 diabetes mellitus. Thirty-six normotensive (24-hour blood pressure <130/80 mm Hg) subjects with inadequately controlled (glycated hemoglobin >7%) type 1 diabetes, without clinically detectable heart disease, were enrolled. After the basal evaluation, insulin therapy was modified to improve glycemic control. Glycated hemoglobin, LV echocardiography, 24-hour blood pressure monitoring, and laboratory tests were repeated after 6 months in all patients and after 12 months in 27 patients. At the basal evaluation, LV anatomy and systolic function were normal in all, and diastolic function was impaired in 14 patients. After 6 months, the mean values of body mass index, 24-hour blood pressure, and LV anatomy and systolic function were unchanged; mean glycated hemoglobin was decreased (p < 0.001), and mean values of diastolic parameters were significantly improved. After 12 months, the mean values of all blood pressure, metabolic, and LV parameters were unchanged. Percent changes of diastolic parameters were inversely correlated with percent changes of glycated hemoglobin, considering changes from the basal to the 6-month evaluation, as well as changes from the 6- to the 12-month evaluation. In conclusion, in normotensive patients with type 1 diabetes, a close relation was found between glycemic control and LV diastolic function, which improves when glycemic control improves. Therefore, diastolic dysfunction can be prevented or reversed, at least partly, by tight glycemic control.