An investigation was made of the pathology of osteochondritis dissecans of young foals and horses with clinical signs of the lesion. A randomly selected material of fetuses and young foals without clinical signs was also examined. It was demonstrated that osteochondritis dissecans is primarily a cartilaginous disease, as previously described in pigs and dogs. Thickening, disturbance of endochondral ossification, degeneration and necrosis of the cartilage were the four main features of osteochondritis dissecans. Cracks and fissures occurred in the degenerated and necrotic parts of the cartilage. This led to formation of cartilage flaps and eventually to loose bodies. It was shown that small pieces of subchondral bone could be ripped off when a cartilage flap was formed. This was one explanation as to why many flaps and loose bodies contained bone in contrast to the findings in pigs and dogs. Endochondral ossification could also take place in the thickened joint cartilage in some cases. Even some loose bodies could undergo endochondral ossification if they were well nourished. Osteochondritis dissecans was often found bilaterally in the knee and hock joint and this was interpreted as an indication that osteochondritis dissecans is a manifestation of a generalized condition called osteochondrosis. Simultaneous occurrence of lesions in joints other than the knee and hock and in several metaphyseal growth plates was another indication of the generalized nature.