To develop and validate a self-administered instrument for measuring functional status in Japanese-speaking rheumatoid arthritis patients. We translated the Stanford Health Assessment Questionnaire (HAQ) into Japanese (original HAQ), and then made a tentative Japanese version of the HAQ (J-HAQ) with culturally appropriate modifications of the arising, eating, and reach category questions. The questionnaire was then administered to 3,763 RA patients (82.6% female; mean age 58.0 years; mean onset age 47.4 years; mean disease duration 10.5 years). This instrument showed excellent internal reliability (Cronbach's alpha = 0.927), with a mean interitem correlation of 0.60. For the arising category question, the J-HAQ asks about arising from a futon in addition to a bed because futons are still common in Japanese culture. Arising from a futon is generally more difficult for disabled individuals than is arising from a bed, so the arising score was higher in the J-HAQ (mean score 0.82) than in the original HAQ (0.48). The average scores for the eating and reach categories were virtually identical for the original HAQ and the J-HAQ, with correlation coefficients of 0.979 and 0.926, respectively. Thus, the overall disability index (average of the scores for all functional areas) was higher in the J-HAQ (0.81) than in the original HAQ (0.76), although the correlation coefficient was high (0.993). The test-retest reliability value (0.92), studied at a 1-week interval, revealed identical disability index scores measured on the 2 occasions. The final version of the J-HAQ is a valid and reliable instrument for measuring functional status in Japanese-speaking RA patients.