The mucopolysaccharidoses (MPS) are a family of lysosomal storage diseases that result from the accumulation of partially catabolized glycosaminoglycans (GAGs) within lysosomes. A characteristic of most affected individuals is radiographic evidence of symmetrical epiphyseal dysplasia, with short stature and degenerative joint disease. Although there is evidence of epiphyseal dysfunction, little is known of the changes that occur at the morphological level. The growth plate of the femoral head was studied by light and electron microscopy in five cats with MPS VI (Maroteaux-Lamy syndrome, arylsulfatase B deficiency) and 12 normal cats. Compared with the normals, the MPS VI cat growth plates exhibited poorly organized proliferative zones, an almost total loss of column formation in the hypertrophic zone, an uneven chondro-osseous junction, a disorganized calcifying cartilage zone, and abnormal or reduced numbers of osteoclasts. By electron microscopy, the cytoplasm of affected cat chondrocytes was filled with membrane-bound vacuoles. Together these findings indicate that the MPS diseases cause major changes in growth plate structure and function.