Polyglutamine expansions in the ataxin-2 gene (ATXN2) cause autosomal dominant spinocerebellar ataxia type 2 (SCA2), but have recently also been associated with amyotrophic lateral sclerosis (ALS). We present clinical and pathological features of a family in which a pathological ATXN2 expansion led to frontotemporal lobar degeneration with ALS (FTLD-ALS) in the index case, but typical SCA2 in a son, and compare the neuropathology with a case of typical SCA2. The index case shares the molecular signature of SCA2 with prominent polyglutamine and p62-positive intranuclear neuronal inclusions mainly in the pontine nuclei, while harbouring more pronounced neocortical and spinal TDP-43 pathology. We conclude that ATXN2 mutations can cause not only ALS, but also a neuropathological overlap syndrome of SCA2 and FTLD presenting clinically as pure FTLD-ALS without ataxia. The cause of the phenotypic heterogeneity remains unexplained, but the presence of a CAA-interrupted CAG repeat in the FTLD case in this family suggests that one potential mechanism may be variation in repeat tract composition between members of the same family.