Self assessment health status questionnaires are increasingly used to measure health status or the effect of treatment in patients with rheumatoid arthritis (RA). Most of these questionnaires measure functional (physical) disabilities. The question arises, however, as to how well self assessment questionnaires reflect the true functional status of patients or whether they only reflect their imaginary functional capacities. How valid is the opinion of patients with RA about their own functional capacity? To answer this question an investigation was performed in 80 patients with RA. Forty Dutch and 40 Belgian patients with RA completed the functional items of the DUTCH-AIMS, the Dutch version of the Arthritis Impact Measurement Scales (AIMS), a self assessment questionnaire specific to arthritis. Their scores on the functional scales were compared with the scores on the same scales completed by two experienced physiotherapists after evaluation of the functional ability of these patients. This was achieved by observing the patients perform the tasks given in the questionnaire. Correlation coefficients between the scores of the patients and the physiotherapists were highly significant for all the scales. No significant differences were found between the patients' and physiotherapists' mean scale scores except for the mobility scale in the Dutch patients. The strength of agreement (Cohen's kappa) of most scale scores of the patients and physiotherapists was substantial. The estimates of the overall functional capacity (the mean of the five scale scores) of the Belgian and Dutch patients show high correlations between the patients and the physiotherapists. It is concluded that patients' opinion about their functional ability is valid in that it is in agreement with their real functional abilities. This study provides further evidence for the validity of the DUTCH-AIMS as a measure of functional disability and health status in Dutch and Belgian patients with RA.