AbstractJoint tissues from patients with rheumatoid arthritis (3), osteoarthritis (3) and normal healthy individuals (2) were examined in the transmission electron microscope and the morphological features of capillaries within 2 mm of the synovial membrane were quantified. Weibel‐Palade bodies (WPB) were significantly increased in capillary endothelial cells in rheumatoid and osteoarthritis compared with normal controls, when quantified by three different methods. WPB were positively correlated with both projections of the luminal plasmalemma and pinocytic vesicles in normal joint and some patients with rheumatoid and osteoarthritis. Pinocytic vesicles and projections were also positively correlate