Lesions observed in the subchondral marrow of metatarsal and metacarpal heads in advanced rheumatoid arthritis are characterized by proliferation of fibroblasts, deposition of collagen, formation of thin-walled vascular channels, infiltrates of macrophages, lymphocytes and plasma cells, and new bone formation. The overlying hyaline articular cartilage is often necrotic; pannus formation is absent. Although the tissue reaction is basically reparative in nature, aggregates of lymphocytes and plasma cells are also observed around small blood vessels in deeper regions of the bone marrow and may point to a primary bone lesion in this disease.