A de novo somatic mutation in the mammalian target of rapamycin (MTOR) has previously been described in one patient with hemimegalencephaly and epilepsy. Here, we present a case of a young girl with megalencephaly and intractable seizures who was found to have an MTOR mutation in multiple cell lineages (p.Cys1483Phe) and, therefore, presumed to be of germline origin. The mutation was detected in peripheral blood DNA by exome sequencing of the patient and her parents, substantiating the utility of this approach for detection of clinically relevant de novo variations.