We describe a family in which infantile Werdnig-Hoffmann disease and adult-onset progressive muscular atrophy both occurred. The possibility of these two diseases developing within the same family by chance is unlikely, and several genetic hypotheses may be put forward to explain the association. We suggest that the molecular pathogenesis of these two subtypes of lower motor neuron degeneration may be linked. The genetic defect in the childhood spinal muscular atrophies has been mapped to chromosome 5q in close proximity to the microtubule-associated protein 1B locus. The association of diseases within this family suggests that chromosome 5q should also be studied in relation to adult-onset familial motor neuron disease.