The glucose transport protein type 1 (GLUT1) deficit causes a chronic brain energy failure. The classic phenotype of GLUT1 deficiency syndrome is characterized by: Mild to severe motor delay and mental retardation; infantile-onset epilepsy; head growth deceleration; movement disorders (ataxia, dystonia, spasticity); and non-epileptic paroxysmal events (intermittent ataxia, periodic confusion, recurrent headaches). During last years the classic phenotype of this syndrome, as originally reported, has expanded. We report the atypical phenotype of a boy with GLUT1 deficiency syndrome, characterized by mild mental retardation and drug-resistant absence seizures with onset at the age of 6 years, without movement disorders nor decrease of head circumference. A prompt diagnosis of this disorder is mandatory since the ketogenic diet might represent an effective treatment.