The most suitable measures to assess the disease activity of rheumatoid arthritis patients treated with slow-acting anti-rheumatic drugs were considered in a prospective study. This was organised across Europe in 12 specialised centres and 282 patients were studied. The patients were all considered to be in need of therapy with a slow-acting anti-rheumatic drug and were studied at the initiation of therapy, and after 3 and 6 months of treatment. There were 215 patients who remained on treatment for 6 months. The most useful measures to assess disease activity were: the number of swollen joints, the number of tender joints, pain, the patients' assessment of response, and ESR. These should form a minimum data set when assessing the activity of rheumatoid arthritis. Some measures such as grip strength, hemoglobin, and the C-reactive protein level showed too much variation between centres and will require considerable standardisation before they can be used across Europe. There were problems in collecting functional data and further work is needed to develop a functional questionnaire available in all European languages with culturally suitable questions.