In this case, we presented the electro-clinical findings in a patient with mental retardation, epilepsy and bilateral posterior agyria-pachygyria. Clinical findings were characterized by frequent tonic generalized and scarcely with myoclonic and absence seizures. Interictal electroencephalography (EEG) showed synchronous and asynchronous delta waves in the posterior region, spike activity predominantly on the right side and fast alpha rhythms. Cerebral magnetic resonance imaging revealed bilateral, symmetric thickened cortex in the parieto-occipital lobes and reduced volume of white matter. These findings were compatible with agyria and pachygyria of posterior regions of the brain. Consequently, in the patient with mental retardation and intractable epilepsy characterized by interictal EEG with posterior focal epileptiform abnormalities, diffuse polyspike-wave paroxysms, bilateral parieto-occipital agyria-pachygyria should be considered as a possible etiology.