This study describes changes in cognitive function in patients who are typically excluded from longitudinal studies due to their inability to complete most standardized batteries. Thirty-three patients with moderate to severe Alzheimer's disease were assessed by means of the Severe Impairment Battery (SIB), Mini Mental State Examination (MMSE), Clinical Dementia Rating (CDR), and measures of functional status. Follow-up evaluations were conducted, and annual rates of change were calculated. Patients with severe cognitive impairment demonstrated rates of progression similar to patients who were moderately impaired at initial evaluation. Rates of decline on the SIB were unrelated to baseline cognitive function, and annual rate of decline in Year 1 was not correlated with disease progression in the subsequent year. This study demonstrates that, with appropriate assessments, severely impaired patients can exhibit a range of performance and rates of decline that are comparable to those of moderately impaired patients. (JINS, 1998, 4, 512–516.)