In 10 patients with severe pain localised over the dorsal aspect of the lunate, the capsule of the wrist joint was exposed and fenestrated over the scapho-lunate ligament. In two cases flat ganglion cysts were found, too small to be palpated before surgery. In the other 8 cases without macroscopically visible changes, histologically, areas of mucoid degeneration and very small ganglion cysts could be seen in the capsule. All patients were pain free after surgery. It is concluded that very small intracapsular cysts in the area of the dorsal scapho-lunate ligament, the most common origin of ganglia, may irritate the posterior interosseus nerve which is located in this area.