The brains of five cases of severe infantile form of Werdnig-Hoffmann (W-H) disease were studied to observe the pathologic changes of sensory neurons and the thalamus. The present study disclosed severe cell loss, chromatolytic degeneration, and empty cell beds of the spinal anterior horn and cranial motoneurons (V, VII, X, XII). Glial bundles were also noted in the anterior roots. In the sensory systems, glial bundles in the posterior roots (2/5), ghost cells in Clarke's column (2/5), and degeneration of the thalamus, mainly in the lateral formation (4/4) were noted. It was demonstrated that not only degeneration of lower motor neurons and glial bundles in the anterior roots, but also degeneration of sensory neurons and thalamus were present in W-H disease. These findings suggested the possibility that W-H disease is a multisystemic disease involving both the anterior and posterior root systems. No sensory involvement was found clinically. Characteristic wrist drop was observed in four cases, two of which also having motor nerve conduction velocity (MCV) delay. On the other hand, MCV of another case without wrist drop was normal. The possibility that wrist drop might be one of the clinical features of peripheral nerve dysfunction was discussed, but further pathologic evaluation of peripheral nerves is needed.