AbstractFirst- and second-degree relatives of people with amyotrophic lateral sclerosis report higher rates of neuropsychiatric disorders, indicating that risk genes may be pleiotropic, causing multiple phenotypes within kindreds. Such phenotypes may constitute a disease endophenotype that associates with disease liability. We have directly investigated cognitive functioning and neuropsychiatric traits among relatives of people with amyotrophic lateral sclerosis to identify potential endophenotypes of the disease. In a family-based, cross-sectional study design, first- and second-degree relatives of people with amyotrophic lateral sclerosis (n= 149) were compared to controls (n= 60) using an in-depth neuropsychological and neuropsychiatric assessment. Subgroup analyses examined the effect of family history andC9orf72repeat expansion status (n= 16 positive carriers). Relatives of people with amyotrophic lateral sclerosis had lower scores on executive functioning, language and memory tasks compared to controls, with large effect sizes observed on object naming (d= 0.91,P= 0.00001) and phonemic verbal fluency (d= 0.81,P= 0.0003). Relatives also had higher autism quotient attention to detail traits (d= −0.52,P= 0.005), lower conscientiousness (d= 0.57,P= 0.003) and lower openness to experience personality traits (d= 0.54,P= 0.01) than controls. These effects were typically larger in relatives of people with familial, rather than sporadic, amyotrophic lateral sclerosis and were present in both gene carrier and non-carrier relatives of probands with aC9orf72repeat expansion. Poorer phonemic fluency and object naming, along with autism and personality traits, are more frequent in relatives of people with amyotrophic lateral sclerosis. Among kindreds carrying theC9orf72repeat expansion, these traits were identified in relatives regardless of their carrier status, suggesting the presence of a disease-associated endophenotype that is not exclusively mediated by theC9orf72expansion.