Osteogenesis imperfecta (OI) and Klinefelter’s syndrome (KS) are genetically determined disorders that affect the skeletal system. is a heterogeneous group of usually autosomal, dominant diseases due to a mutation in one of the two genes that encode procollagen-I [1-5]. It shows great variability of gene expression, which suggests the role of unidentified factors that segregate independently and modulate the final phenotype. The X chromosome contains genes that are known to determine skeletal structure. Effects of testosterone and estrogens on the skeletal system are also well known [6,7]. Klinefelter’s syndrome is the most common cause of testicular failure with impairment of spermatogenesis and, to a lesser extent, of testosterone production. It is the most common sex-chromosome disorder affecting 1 in 500 men in all ethnic groups, and is due to presence of one or more additional X chromosomes or to structural abnormalities in the X chromosome. Impaired bone mineral density (BMD) and osteoporosis are common in the hypogonadotropic hypo­gonadism that is typical of KS. We describe a patient in whom both OI and KS are present.