We describe three members each of two families presenting with a hereditary form of lower motor neuron disease with adult onset and rapid progression and compare their pathological and clinical features with hereditary lower motor neuron disease with adult onset, as described in the literature. No involvement of upper motor neurons was found either clinically or pathologically. Disease progression was rapid, and the majority of patients died from respiratory failure within 1-5 years after onset of disease. On pathological examination of the spinal cord we found ballooned neurons, neuronophagia and gliosis in family A, which have been regarded as characteristic pathological features of infantile-onset spinal muscular atrophy (SMA). In family B specific neuronal changes were observed that also occur in patients with amyotrophic lateral sclerosis (ALS). An autosomal dominant mode of inheritance would seem likely in both families. In family A the pathological findings and the clinical presentation with symmetrical proximal limb weakness show similarities with autosomal dominant SMA. Based on the finding of pathological features in family B that also occur in ALS, together with the distal asymmetrical muscle weakness and bulbar signs and a high age at onset we hypothesize that the members of family B suffered from familial ALS. The disease forms in both families in our opinion further broaden the spectrum of motor neuron disease.