There is evidence that some elderly patients with chronic schizophrenia experience marked impairments in cognitive functioning. Assessment of these patients may be difficult with traditional neuropsychological measures. The purpose of the present study was to determine if cognitive functioning could be validly assessed with the Alzheimer’s Disease Assessment Scale—Late Version Cognitive factor score (ADAS-L Cog) in patients whose scores on the Mini-Mental State Examination (MMSE) reflect profound cognitive impairment. Patients with MMSE scores from 0 to 10 were selected from a larger database. Neuropsychological instruments designed for the assessment of mild to moderate dementia were found to be inadequate in this profoundly impaired population, due to floor effects. In contrast, there was a significant relationship between ADAS-L scores and several criterion measures, including the MMSE (R=−.71, P<.001), the Social Adaptive Functions Evaluation (SAFE) social functions scale (R=.47, P<.001), and the negative symptom total score of the Positive and Negative Syndrome Scale (PANSS) (R=.412, P<.001). The MMSE was somewhat less strongly correlated with both social functions (R=−.401, P<.001) and the negative symptom total score of the PANSS (R=−.366, P<.001). These results suggest that cognition can be reliably and validly assessed with instruments such as the ADAS-L that are designed for the assessment of severely impaired patients.