A scale was developed that appears to be a reliable and sensitive indicator of differences and changes in the quality of life of patients with Alzheimer's disease (AD) as the disease progresses. The instrument was generated from interviews with spouses and/or caregivers of AD patients, and was designed to be completed easily by the caregiver. The bipolar analog scale was based on the daily behavior of AD patients and was developed in three steps involving interviews with caregivers (step I), testing of questionnaires, and validation of the instrument. At step II, four iterative trials, each with a different version of the questionnaire, were performed for validation. The final form of the Progressive Deterioration Scale (PDS) contained 27 quality-of-life factors and was cross-validated in a second, separate study. The Global Deterioration Scale (GDS) for cognitive function was used as the external criterion for the analysis of the quality-of-life scales. Statistical analysis indicates that the PDS provides a reliable, valid, and sensitive measure of changes in the quality of life as AD progresses. The PDS achieved 80% overall accuracy in discriminating non-AD elderly patients from patients in the early, middle, and late stages of AD. This scale may provide a useful means for clinical evaluation (phase III) of potential medications to alleviate the symptoms of AD.