Objective: To investigate the prevalence and nature of cognitive changes associated with sporadic amyotrophic lateral sclerosis (ALS) using a large scale study. Methods: Consecutive patients with sporadic ALS (n = 279) underwent comprehensive neurologic evaluation and neuropsychological testing. Testing data from normal controls (n = 129) were used for classification and comparison purposes. Results: On non-motor, non-speed-dependent tasks, 51%of patients with ALS had evidence of cognitive impairment compared to 5%of controls. Cluster analysis suggested four patient subgroups: 49%intact, 32%with mild impairment, 13%with moderate impairment, and 6%with severe impairment. Forty-one patients (15%) met criteria for frontotemporal dementia (FTD). ALS patient subgroups, excluding the intact group, performed significantly lower on tests of executive function and memory than normal controls. Patients with more severe disease also had deficits in confrontation naming. Although memory function declined with increasing severity of overall cognitive impairment, only two patients had the severe memory loss typical of Alzheimer disease. Cognitive impairment was correlated with clinical measures of word-finding, phrase length, and motor programming. Cognitive impairment was not correlated with depression scores or severity or duration of motor or bulbar symptoms. Patients with bulbar vs limb-onset ALS were not different in either level of impairment or pattern of performance. Conclusions: These data confirm the presence of cognitive impairment in 50%of patients with ALS and particularly implicate executive dysfunction and mild memory decline in the disease process. More severe impairment occurs in a subset of patients with ALS and has features consistent with FTD. Objective: To investigate the prevalence and nature of cognitive changes associated with sporadic amyotrophic lateral sclerosis (ALS) using a large scale study. Methods: Consecutive patients with sporadic ALS (n = 279) underwent comprehensive neurologic evaluation and neuropsychological testing. Testing data from normal Results: On non-motor, non-speed-dependent tasks, 51% of patients with ALS had evidence of cognitive impairment compared to 5% of controls. Cluster analysis suggested four Forty-one patients (15%) met criteria for frontotemporal dementia (FTD). ALS patient subgroups, excluding the intact (15%) met subgroups: 49% intact, 32% with mild impairment, 13% with moderate impairment, and 6% with severe impairment group, performed significantly lower on tests of executive function and memory than normal controls. Patients with more severe disease also had deficits in confrontation naming. Cognitive impairment was correlated with clinical measures of word-finding, phrase length, and motor programming. Cognitive impairment was not correlated with depression scores or Patients with bulbar vs limb-onset ALS were not different in either level of impairment or pattern of performance. Conclusions: These data confirm the presence of cognitive impairment in 50% of patients with ALS and particularly implicate executive dysfunction and mild memory decline in the disease process. More severe impairment occurs in a subset of patients with ALS and has features consistent with FTD.