The validity and reliability of the Barthel index were studied in 60 geriatric patients, 87 stroke patients, and 102 patients with hip fracture, using a factor analysis methodology which explicitly accounts for the ordinal nature of the scoring on each item. The findings substantiate that the Barthel index is unidimensional among stroke patients, but not among geriatric patients or patients with hip fracture. In the latter two groups, one factor related to mobility, the other to bodily functions. A sum-score to characterize geriatric and hip fracture patients does not take into account the complex structure of the Barthel index.