Fragmentation of the medial malleolus of the tibia was found radiographically in 5 canine tarsi which did not have evidence of osteochondrosis of the medial trochlear ridge. An additional 5 tarsi were found where both medial malleolar fragmentation and osteochondrosis of the medial trochlear ridge were present. Radiographic evidence of degenerative joint disease was present in 3 of 5 dogs with medial malleolar fragmentation alone, and 5 of 5 dogs with medial malleolar fragmentation and medial trochlear ridge osteochondrosis. Eight of the 9 dogs were Rottweilers. Considering the sites of occurrence of osteochondrosis in other species, the authors propose that medial malleolar fragmentation could be secondary to osteochondrosis of the medial malleolus. Osteochondrosis of the medial malleolus has not been previously reported in dogs. Histological examination of the medial malleolar fragmentation was unavailable because surgery was not performed, therefore the hypothesis that the medial malleolar fragmentation is due to osteochondrosis was not proven.