Cerebral developmental malformations are increasingly recognized as a cause of epilepsy. Magnetic resonance imaging (MRI) has advanced our understanding of these disorders and their relation to epilepsy. We report the occurrence of discordant unilateral heterotopia and epilepsy in monozygotic twins. The affected individual developed intractable focal seizures at age 16 years. Mild cognitive difficulties had been present in early life. Evaluation showed right hemisphere EEG epileptogenic abnormalities, and the MRI scan showed massive right hemisphere heterotopia. EEG and MRI examinations in the patient's twin brother were normal. These findings suggest that the development of some developmental brain malformations and epilepsy is strongly influenced by nongenetic factors such as an environmental insult.