The authors probed the associations between clinical diagnoses and independent research measures of cognitive, behavioral, and electroencephalographic (EEG) changes in hospitalized older patients and investigated the contribution of medical illness to deterioration. Patients (N=96; 47 of whom were hospitalized during the course of 1 year; 12 diagnosed with delirium) received tests of cognitive and physical functioning and the Cumulative Illness Rating Scale, specific neuropsychological tests, and a two-channel EEG. Delirium was associated with independent measures of cognitive decline and EEG slowing. Hospitalization was associated with deterioration in functional status during the year, whether or not patients showed delirium. Results suggest that medical illness leading to hospitalization can contribute significantly to deterioration in self-care, and, when it is associated with delirium, to deterioration in cognitive performance and cerebral activity over a period of 1 year.