The cartilage-pannus junction has been studied in multiple sections from 23 rheumatoid joints. Changes suggesting a metaplastic reaction of the articular cartilage, termed transitional fibroblastic zone, were commonly found in hips and knees, but were rarely present in metatarsophalangeal joints, in which an invasive pannus with cartilage degradation in close association with inflammatory cells was seen. Thus when multiple sections from rheumatoid joints were examined a transitional fibroblastic zone was found in 1/15 (7%) sections from metatarsophalangeal joints compared with 29/57 (51%) and 15/48 (31%) sections from knee and hip joints respectively. In contrast, an invasive pannus occurred in 11/15 (73%) sections from metatarsophalangeal joints compared with 22/57 (39%) sections from knees and 19/48 (40%) sections from hips. These findings led to the suggestion that this pathological variation between different joints may explain the predominance of erosive change in small joints as compared with joint space narrowing with secondary osteoarthritis found in large joints in rheumatoid arthritis. Inappropriate comparisons between different joints may in part explain the variation in findings of previous histopathological studies.