Hereditary motor and sensory neuropathy associated with agenesis of the corpus callosum (OMIM 218000) is an autosomal recessive disease of early onset characterized by a delay in developmental milestones, a severe sensory-motor polyneuropathy with areflexia, a variable degree of agenesis of the corpus callosum, amyotrophy, hypotonia, and cognitive impairment. Although this disorder has rarely been reported worldwide, it has a high prevalence in the Saguenay-Lac-St-Jean region of the province of Quebec (Canada) predominantly because of a founder effect. The gene defect responsible for this disorder recently has been identified, and it is a protein-truncating mutation in the SLC12A6 gene, which codes for a cotransporter protein known as KCC3. Herein, we provide the first extensive review of this disorder, covering epidemiological, clinical, and molecular genetic studies.