There is no agreed single measure of physical disability for use either clinically or in research. It is argued that acceptance of a single standard measure of activities of daily living (ADL) might increase awareness of disability, improve clinical management of disabled patients, and might even increase acceptance of published research. The Barthel ADL Index is proposed as the standard index for clinical and research purposes. Its validity, reliability, sensitivity, and utility are discussed. The Barthel Index is as good as any other single simple index, and should be adopted as the standard against which future indices are compared. The temptation to use variations on the standard Barthel Index should be resisted.