Lissencephaly (agyria) is a cortical dysplasia associated with a disturbance in the migration of neocortical neurons. Because of abnormal sonographic findings in the brain of a 24-week fetus, we carried out serial magnetic resonance imaging (MRI) examinations which raised the suspicion of isolated lissencephaly. In the second trimester, an area of damage depicted as low-intensity signals was identified by MRI inside the cortical parenchyma. In the same cortical area, agyria was detected later in pregnancy, and postnatally calcification was observed by computerized tomography (CT) scan. We believe that ultrasound and targeted MRI examination can improve our understanding of some disorders of neuronal migration and that earlier diagnosis is feasible if both methods are employed and interpreted by sound criteria.