The Northwick Park activities of daily living (ADL) index was used to assess functional disability in rheumatoid arthritis patients. Assessments were carried out by two pairs of observers and inter-observer variation was studied. No significant differences were found between observers for the total independence score and the total time taken to complete individual tasks. A significant difference was found between observers in Pair A with regard to the total assessment time, as one member from this pair took consistently longer to complete the assessment than the other. Although this trend was also apparent in Pair B, this was not significant. The modified ADL index is recommended for routine assessment in the general treatment and rehabilitation of patients with rheumatoid arthritis.