The structure of lymphatic capillaries (LC) of the synovial membrane (SM) from patients with rheumatoid arthritis and juvenile idiopathic arthritis obtained by synovectomy was investigated by transmission electron microscopy. This method allows comparison of the structure of the same vessel under light and electron microscope and clear differentiation between lymphatic and blood capillaries and venules. Synovial LC were localized in the subintimal connective tissue of the SM in the vicinity of venules. The shape of some LC was irregular, suggesting edema of the interstitium. Lymphatic endothelium has extremely attenuated cytoplasm with the exception of the perinuclear region. Many nuclei of endothelial cells had distinct nucleoli. The basal lamina was discontinuous. The walls of LC showed close connection with the interstitium represented by anchoring filaments that were attached to the endothelial cells and to the surrounding connective tissue. In some LC connective tissue appeared to be disconnected from endothelium and gaps between their walls and the interstitium were seen. Mononuclear cells were accumulated adjacent to some LC. Specialized interendothelial junctions (endothelial microvalves) were observed in the LC walls. Their structure and function in the migration of cells and debris from synovial interstitium into LC lumina in rheumatoid arthritic synovium deserves further investigation. In the lumina of some of the LC lymphocytes, monocytes, macrophages, cell debris and enlarged endothelium were observed. Accumulation of such material may cause obstruction of tiny LC. We suggest that reported alterations of the fine synovial lymphatic vessels can contribute to the progression of the inflammatory process to chronicity.