Multiple biopsy specimens from various parts of the synovial membrane were sampled under direct vision during arthroscopic surveying of the inflamed knee joints of 12 patients with various arthritides. Considerable variation in the macroscopic signs of inflammatory activity was found within the single joint. However, there was a highly significant correlation (P < 0.001) between local macroscopic signs of inflammatory activity and microscopic signs (by immunohistochemical analysis of the corresponding tissue sample). The profound intraarticular variation in inflammatory activity was seen irrespective of clinical diagnosis. An immunohistologic pattern comprising foci of T helper cells, associated immunoglobulin-bearing cells, and HLA-DR-expressing dendritic cells was not specific for rheumatoid arthritis or any other diagnosis, but was found in the biopsy samples from sites that macroscopically demonstrated maximal inflammatory activity.