The c9orf72 repeat expansion (c9 or c9orf72 RE) confers a survival disadvantage in amyotrophic lateral sclerosis (ALS); its effect on prognosis in frontotemporal dementia (FTD) remains uncertain. Data on prognostic factors in c9orf72 RE disorders could inform patient care, genetic counseling, and trial design. OBJECTIVE To examine prognostic factors in c9ALS, c9FTD, c9ALS-FTD, and atypical phenotypes.