Over a period of three years, a diagnosis of fragmented medial coronoid process of the ulna was confirmed by arthroscopy in 263 dogs, in 69 of which both elbow joints were affected, giving a total of 332 affected joints; 39 of the joints were also affected by osteochondrosis dissecans of the trochlea of the humerus. More than half of the dogs (51·7 per cent) were over one year old when first examined. In 259 (78 per cent) of the joints there were radiographic changes of secondary osteoarthrosis of different severity, but the other 73 (22 per cent) had no signs of osteophytosis except for a mild sclerosis. Among these 73 there were 14 joints in 13 dogs between 19 and 36 months old and 11 joints in 10 dogs over three years old. A further 12 joints in 11 dogs over three years old showed only mild arthrosis. In five of the 11 joints of the dogs over three years old which had no radiological signs of arthrosis, the fragment was already dislocated. Among the dogs over 18 months old with no arthrosis and the dogs over three years old with mild arthrosis, the most commonly affected breeds were the Bernese mountain dog, rottweiler and boxer.