Glucose transporter type 1 deficiency syndrome is a neurometabolic encephalopathy characterized by movement disorders, intractable seizures, and acquired microcephaly. Two girls and a boy between the ages of 3 and 15 years were included in this study. The main clinical manifestations were seizure, ataxia, global developmental delay, and acquired microcephaly. The most common electroencephalographic finding was interictal focal or generalized epileptiform discharges. The cerebrospinal fluid to blood glucose ratio was determined to be low (0.35 and 0.40). Two cases had heterozygous de novo mutations and one had microdeletion of SLC2A1. All cases were treated with a ketogenic diet (KD) and were seizure-free in the sixth month of the diet. KD also improved ataxic gait, language skills, and behavioral disturbances. Inconsistency was demonstrated between electroencephalography findings and seizure semiologies detected in patients with GLUT1DS, and KD was found to be most effective for seizures and less effective for ataxia, language skills, and behavioral disturbances.