A working group of ten rheumatology institutes (UNIREUM) was formed in Italy to promote multicenter therapeutic studies on rheumatoid arthritis. The Functional Disability Index (FDI) of the Health Assessment Questionnaire (HAQ) was chosen as a measure of disability. This paper reports the results of a multicenter study to validate an Italian translation of the instrument and to assess its reliability and validity. Two questions were modified to achieve cross-cultural equivalence. Back-translation into English showed agreement with the American instrument. Reproducibility was high: the test-retest correlation coefficient was 0.989 and ranged from 0.81 to 0.99 for the centers taken separately. Validity was confirmed by a correlation coefficient of 0.95 between the patient self-attributed and the physician-attributed FDI scores. We conclude that our Italian version of the HAQ FDI is a reliable and valid self-administered instrument.