Three newborn siblings presented with generalized weakness, asphyxia, facial diplegia, and external ophthalmoplegia. Electrophysiological testing showed inexcitability of motor and sensory nerves and myographic signs of denervation. Nerve biopsies and postmortem examination showed loss of myelinated fibers and axonal damage in sensory and mixed nerves. Many spinal motor neurons were chromatolytic although their number was normal. Molecular genetic investigations revealed a homozygous deletion of the survival motor neuron (SMN) gene and a loss of markers Ag1-CA and C212 in the paternal haplotype. These findings are consistent with the diagnosis of an unusually severe type of spinal muscular atrophy. Given the large extent of the deletion, it must be considered that the unusual severe phenotype with involvement of brainstem nuclei and afferent nerves might also be due to changes of yet unknown genes neighboring the SMN gene.