The aim of this study was to investigate the influence of metabolic control on quantitative electroencephalographic (EEG) abnormalities in young diabetic patients. We compared quantitative EEGs of 44 insulin-dependent diabetic patients with age-matched controls. Furthermore, differences in EEG variables of diabetic patients and controls were correlated with the age of onset and duration of diabetes, hemoglobin A1c concentrations, and episodes with severe hypoglycemia. We found a correlation between high values of hemoglobin A1c and decreased relative power of the alpha band. Patients with previous ketoacidotic episodes had increased relative power of the delta/theta band and decreased relative power of the alpha band. Patients with hemoglobin A1c values less than 8.5% exhibited slowing of background activity compared with normal controls. We conclude that metabolic control influences the EEG and that improvement of glucose metabolism is an important factor in avoiding EEG abnormalities in young diabetic patients.