Three syndromes are presented in which major midline malformations of the central nervous system were associated with characteristic somatic and neurologic features in 2 or more sibs. The malformations may be suspected on clinical grouds but require confirmation by pneumoencephalography. In 3 French-Canadian sibships from the Saguenay-Lac St. Jean area of Quebec, patients with areflexia, muscular wasting and slowly progressive weakness in a paraparetic distribution were proved to have agenesis of the corpus callosum and anterior horn-cell disease, a syndrome not previously described. In another family, mental retardation, ataxia and episodic hyperpnea were associated with agenesis of the cerebellar vermis in 4 sibs. In yet another French-Canadian family, atrophy of the cerebellar vermis was associated with mental retardation, ataxia and a mild pyramidal syndrome. Because malformations of this nature are usually considered sporadic or multifactorial in origin, recognition of these specific clinical syndromes with probable autosomal recessive inheritance is important from the point of view of genetic counseling and prevention.