The majority of measurement scales used to evaluate outcome in rehabilitation are ordinal in nature and consequently statistically valid assessments of change are difficult to make. The Functional Independence Measure (FIM) can be weighted to possess interval properties, potentially allowing more accurate analysis of change. In this study the FIM was compared to the Barthel Index (BI) to determine its validity, reliability and ease of use in two groups of 25 patients undergoing neurorehabilitation. The FIM was considered to be more valid than the BI, and equally reliable in the assessment of disability. When the two disability scores were compared using subjective and objective assessment the agreement between them was comparable, although neither was high.