Ununited anconeal process, part of the elbow dysplasia, is a common orthopedic developmentaldisorder in young, fast growing dogs of large and chondrodystrophic breeds. Proximal ulnar osteotomyis one of the possible treatments, provided that the animal is not adult yet. This intervention aims torestore the length proportion between the radius and the ulna, and thereby eliminating the upwardforce, which causes the ununited anconeal process. By performing the osteotomy, the olecranon cantilt, allowing the ununited anconeal process to fuse and resolving the lameness. When a proximal ulnarosteotomy is successfully performed without additional articular surgery, less osteoarthrosis will beformed and a better long-term result can be expected. In this article, the treatment by ulnotomy in ayoung Neapolitan Mastiff and its results are discussed.