OBJECTIVE Evaluation of the clinimetric properties of the Barthel Index. DESIGN To measure disabilities in activities of daily living (ADL) and mobility by independent observers. SETTING Clinical and outpatient departments of Neurology of the University Medical Centre, Amsterdam. PATIENTS AND METHOD In 35 patients of the clinical and 25 patients of the outpatient department the stroke disabilities were assessed independently by three observers. The Barthel Index was evaluated in terms of score agreements, homogeneity, and construct validity. RESULTS There was an excellent concordance of total scores (mean kappa = 0.88; range 0.85-0.90) and single item scores (range mean values of kappa = 0.82-1.00). Repeated analyses, after exclusion of patients with maximum total scores, did not result in a significant reduction of these kappa values. The Barthel Index was a highly homogeneous scale (Cronbach's alpha = 0.96). All items contributed to this reliability in a balanced way. Factor analysis showed that the items on the scale described one common underlying trait. This factor explained 81% of the score variance. CONCLUSION The Barthel Index is a sound instrument to measure disabilities in ADL and mobility. The scale is suitable for use in both patient care and research.