Fifty surgically removed synivial membranes from large joints of patients with rheumatoid arthritis were examined histologically from 10 different areas. The diagnostic reliability and the histologically graded basic and actual activity were estimated. All 3 parameters differed considerably within the individual synovial membrane. To assess the diagnostic value of synovial needle biopsies the results were compared with those of simulated biopsy cylinders. The morphological results gained from the biopsy cylinders were much poorer, showing that needle biopsies are limited diagnostic value.