Osteoporosis in aging women and men is recognized as an increasingly important disorder because of the advancing age of our population and the enormous costs, both monetary and social, involved in the aftermath of bone fractures. Genetic factors are considered to play an important role in the establishment of bone mass. During the past decade, major advances have occurred in our understanding of one genetic cause of osteoporosis, osteogenesis imperfecta (OI). This disorder, genetically and clinically heterogeneous, is the result of mutations affecting the pro-α(I) and pro-α2(I) chains of type I collagen. OI is the prototype for heritable osteoporosis. The phenotype of the mild form of OI, type I, overlaps with that of a familial type of idiopathic osteoporosis. Osteoporosis in this latter group may cluster in families, suggesting a genetic background. Also, it frequently appears before menopause and is associated with mild scoliosis and joint laxity. A limited number of reports document type I collagen defects similar to those in mild OI patients. The incidence of collagen mutations in idiopathic familial osteoporosis is uncertain at this time.