We report on two brothers with partial agenesis of the corpus callosum and seizure disorder presumably related to ectopic grey matter. Development of both patients was characterized by psychomotor retardation and focal epileptic seizures. Genetic examination revealed normal karyotypes. One brother showed a remarkable focal miniature spike and wave periodicity constantly observed on sequential EEG records. Magnetic resonance imaging revealed partial agenesis of the corpus callosum and ectopic grey matter. Prior computerized tomography failed to visualize ectopic grey matter and overestimated corpus callosum agenesis. The heterotopic grey matter, isolated from the surrounding inhibiting influences, is the most probable source of focal seizures and the partial connection of both hemispheres may explain secondary generalization of seizures.