The genetic basis of Developmental Dysphasia is discussed with precise neuropsychological descriptions of 11 cases in six families. In these cases the coexistence in the same family of completely normal and severely impaired siblings suggests a genetic rather than a familial socio-linguistic cause. The possibility of father-son transmission eliminates X-linked inheritance. The results are discussed with a reappraisal of the literature and seem to emphasize the genetic basis, possibly, with autosomal dominant transmission in some cases of Developmental Dysphasia.