The junction between pannus and cartilage was examined in the rheumatoid joint. Three types of cartilage-pannus junction were observed. In one, proliferating small blood vessels, surrounded by highly cellular infiltrates, penetrated deeply into the cartilage. Degeneration of the cartilage was observed around the cellular accumulations. In the second, phagocytic and fibroblastic cells invaded the cartilage. In the third, fibrous pannus overlay the cartilage. Lysis of cartilage by infiltrating cells appeared to be a major cause of cartilage erosion in rheumatoid arthritis.