This study examined whether the presence of cognitive impairment identifies distinct subgroups in patients with ALS. The study longitudinally assessed 192 ALS patients and healthy controls using neuropsychological tests over 6-month intervals. The results showed that (1) patients with executive impairment at baseline had reduced survival and some developed dementia, (2) most with normal cognition at baseline remained so, and (3) all new executive impairment was preceded by subtle changes. The study concludes distinct cognitive subgroups exist in ALS and cognition is a useful clinical biomarker.