Osteoporosis is defined as 'too little normal bone', the disorder being rarer in children than adults. The varied forms in childhood can be classified as those secondary to some other disease and primary forms of the disorder which include the genetically determined osteogenesis imperfecta types and idiopathic forms of osteoporosis. A plea is made for greater clinical application in attempting to discriminate differing forms of these primary disorders. Osteogenesis imperfecta it is argued is a heterogeneous condition with the evidence favouring both dominantly and recessively transmitted forms in different kindreds. Another possible osteogenesis imperfecta variant is characterized by dwarfing, scoliosis and peculiar cystic lesions of the proximal humeri. Idiopathic juvenile osteoporosis is a term reserved for the acute osteoporosis beginning in the immediate prepubertal years and may differ in its cause from idiopathic osteoporosis beginning rather earlier in childhood. It is emphasized that immobilization osteoporosis is of very great importance and may become superimposed upon other osseous dysplasias. A complete understanding of these conditions will be helped by elucidation of the basic underlying defects in collagen and other constituents of bone matrix.