Hereditary multiple exostoses/osteochondromas, also known as diaphyseal aclasis and osteochondromatosis, is an autosomal dominant condition characterised by the development of multiple osteochondromas.

According to the WHO classification of soft tissue and bone tumours (5th edition), the diagnostic criteria are ≥2 radiological osteochondromas at the juxtaepiphyseal region of the long bones and positive family history and/or EXT gene germline mutation.