EXT1/EXT2-CDG (Multiple cartilagineous exostoses, hereditary multiple osteochondroma (MO); OMIM 133700/133701) are common defects of O-xylosylglycan glycosylation. The diagnostic criteria are at least two osteochondromas of the juxta-epiphyseal region of long bones with in the majority of cases a positive family history and/or mutation in one of the EXT genes. The authors report data on clinical symptoms and complications of 23 patients (from 16 families), discussing the family history, age of diagnosis, new clinical and molecular data. Fifteen mutations and large deletions, of which nine are new, were detected in the EXT1 and EXT2 gene by sequence analysis, FISH and MLPA analysis.