Destruction of articular cartilage of rheumatoid knee joints proceeds by erosion with pannus from periarticular tissues. The subchondral changes, however, are not understood well. In the present study, cartilage with subchondral bone of the patello-femoral joints from 20 rheumatoid knees taken during total knee replacement was examined in sections stained with hematoxylin-eosin and safranin-O, as well as by immunohistochemical techniques (peroxidase anti-peroxidase method).Alterations of the articular cartilage and subchondral bone plate were graded into 4 stages: A) Partial destruction of the subchondral bone plate, presence of intramedullary granulation tissue, and concomitant decrease in cartilage matrix staining. B) Numerous erosion sites at the osteochondral junction arising from intramedullary granulation tissue. C) Widespread and multiple invasion by the granulation tissue, but cartilage still remaining. D) Almost total loss of cartilage, replaced with granulation tissue.The osteochondral junction was destroyed by invasion by granulation tissue which proceeded in parallel from both articular cartilage and the intramedullary bone space. Lysozyme was positive on monocytes and fibroblasts at the osteolytic area of the osteochondral junction. S-100 protein was positive on monocytes corresponding to chondrocytes in the same area, where they may take account for lytic function.It was also confirmed histologically that the subchondral bone has high turnover changes.