Changes of the synovium are in integral part of osteoarthritis. Most authors regard these changes as secondary to cartilage degeneration. However, synovitis is a very early feature in osteoarthritis, and increased knowledge of changes in the chemical composition of the synovia focused our attention on the histologic characteristics of osteoarthritic synovium. Light and electron microscopic studies of the synovial membrane from patients with osteoarthritis and rheumatoid arthritis of the hip joints were performed. Two distinct types of osteoarthritic synovitis were observed: an early proliferative from characterized by venous stasis with edema, free erythrocytes and hemosiderine deposits in the interstitial tissue, that suggested increased capillary permeability. In the late form, fibrous synovitis, the microscopic picture was dominated by dense fibrous tissue. Synovium from rheumatoid arthritis showed the same vascular changes as in osteoarthritis. In addition, the histologic picture was characterized by severe inflammatory changes. In osteoarthritis the signs of inflammation were moderate or absent. The development from proliferative into fibrous synovitis is probably the result of long-standing chronic venous insufficiency. The essential feature of osteoarthritic proliferative synovitis--venous stasis with increased capillary permeability--correlates well with most of the known changes in the composition of the synovial fluid.