Pseudoachondroplasia (PSACH) (OMIM#177170) and multiple epiphyseal dysplasia (MED) are separate but overlapping osteochondrodysplasias. PSACH is a dominantly inherited disorder characterized by short-limb short stature, loose joints, and early-onset osteoarthropathy. The diagnosis is based on characteristic clinical and radiographic findings. Only mutations in the cartilage oligomeric matrix protein (COMP) gene have been reported in PSACH, and all family studies have been consistent with linkage to the COMP locus on chromosome 19. Multiple epiphyseal dysplasia (MED) is a relatively mild chondrodysplasia but like PSACH, MED causes early-onset joint degeneration, particularly of the large weight-bearing joints. Given the clinical similarity between PSACH and MED, it was not surprising that the first MED locus identified was the COMP gene (EDM1). Mutations causing MED have now been identified in five other genes (COL9A1, COL9A2, COL9A3, DTDST, and MATN3), making MED one of the most genetically heterogeneous disorders. This article reviews the clinical features of PSACH and MED, the known mutations, and the pathogenetic effect of COMP mutations on the cartilage extracellular matrix. © 2002 Wiley-Liss, Inc.