Three siblings with a recently recognized form of familial arthropathy were studied clinically and by light and electron microscopy of the synovium. On light microscopy, the synovium from all 3 children showed synovial hyperplasia, necrotic villi, deposition of eosinophilic and PAS-positive material, and large numbers of multinucleated giant cells. Electron microscopy of the synovium in one patient revealed giant cells with profuse rough and smooth endoplasmic reticulum. Some phagocytic cells contained degenerated collagen in vacuoles or invaginations. There was granular and fibrinlike material on the synovial surface, but no amyloid. This symmetrical arthropathy now reported in three families appears to have characteristic histological appearance of synovium that allows its identification as distinct from other known joint diseases.