Focal spinal muscular atrophy evolved rapidly in two German Shepherd pups 2 weeks after birth. By 1 month, the male and female littermates had developed a valgus deformity of the right forelimb, a flexed carpus, and arm and forearm muscle weakness and atrophy. In the male, less severe left forelimb weakness and wasting also occurred, and bilateral forelimb denervation potentials were recorded. Postmortem study of the male revealed asymmetric loss and degeneration of motoneurons in the cervical spinal cord intumescence. Degenerating neurons appeared vacuolated or chromatolytic. Chromatolysis was often peripheral and resulted from dispersion and loss of the free and attached ribosomes which normally from Nissl bodies. Although this focal neuronopathy was unlike motoneuron diseases described previously in animals, it resembled the asymmetric and unilateral, benign spinal muscular atrophies found in humans.