Indexes of functional disability are being used increasingly to rate the status of patients studied in clinical research or treated in clinical practice. To determine why so many indexes have been developed and to evaluate their scientific quality, we reviewed the construction and other attributes of 43 indexes that offer ratings for activities of daily living. The six most prominent problems, and some proposed solutions, are as follows: Ratings for the magnitude of performed tasks will be misleading unless the patient's effort or collaboration is suitably considered. Each patient's preferences should be sought to determine which types of disability are the most important goals of therapy. Special transition indexes should be developed if subtle or overt changes are not discerned from the repeated use of single-state indexes. Hierarchical scale arrangements can avoid the loss of descriptive power that occurs when multiple variables are aggregated merely as summations. Documentary evidence can be required to demonstrate the anticipated achievements of an index. New indexes can be constructed if the high statistical "reliability" and "validity" of established indexes are not accompanied by satisfactory clinical "sensibility."