Megalencephaly means an increased size or weight of a generally well-formed brain. It is a feature of a heterogeneous group of mostly familial human disorders with prenatal or early childhood onset. Seizures, motor deficits, mental retardation or milder cognitive impairment are sometimes present. This review discusses idiopathic megalencephalies with regard to possible etiology and treatment opportunities. Idiopathic megalencephalies with neurological deficits as well as unilateral megalencephaly are hypothesized to be caused by disturbances of proliferation, survival or migration of neurons in the brain. The current knowledge of postnatal and adult generation of neurons and survival of adult-borne neurons is reviewed. We show an example of how a genetic potassium channel dysfunction causes not only temporal lobe epilepsy, but also postnatal progressive megalencephaly in a mouse model. We also summarize novel data on neuro-protective effects of the antiepileptic drug carbamazepine in the treatment of brain overgrowth. Findings propose that potassium ion channelopathy may underlie disease for a group of infants or young children displaying idiopathic megalencephaly and early onset epilepsy or episodic ataxia type 1. Carbamazepine's remarkable protective effects on the neuronal plasticity in the hyperexcitable state should be further studied, and maybe this drug should be considered more in treatment of temporal lobe epilepsy and megalencephaly.