Whereas the histopathologic picture of the synovial membrane in rheumatoid arthritis varies widely and that in osteoarthritis displays a small range of changes, light- and transmission electron microscopic studies of synovial sections from both disease entities complement each other and assist reciprocally to corroborate the observed changes. Beyond that comparative survey of the ascertained histopathologic features and their correlation with clinical observations disclose that a study of a larger material of specimens permits with some limitations to infer the nature of the joint disease.