Growth deficiency is a key feature of severe Osteogenesis Imperfecta (OI) and a frequent feature of mild to moderate forms of the disease. The reason that children with OI are short is not fully understood. We do know that details such as the number of fractures suffered or the type of OI do not fully explain the short stature of OI. Growth patterns have been defined for children with OI Types I, III, and IV. At about 12 months of age, children with Types III and IV OI demonstrate a predictable plateau of their linear growth rate. Type IV OI children begin to resume a normal growth rate at about age four to five years, but they will not "catch up" to a normal height, as they have "lost" a significant period of growth. The plateau usually continues for children with Type III OI. The reason for this growth plateau is unknown. There have been no studies which evaluate the growth of OI children in this age range. Our previous studies of growth in OI children have begun at age 5 years.