Osteogenesis imperfecta (OI) and X-linked hypophosphataemia (XLH) are rare genetic diseases, which lead to childhood-onset bone fragility, low-trauma fractures and limb deformities. OI occurs as a result of impaired type 1 collagen synthesis at different stages, depending on the type of a genetic mutation, which leads to bone strength impairment. In most cases OI is a disorder with an autosomal dominant inheritance. However, there are also cases of autosomal recessive inheritance. To date, 16 types of OI are distinguished, with type 2 being the most severe due to 100% mortality rate in neonatal and perinatal periods. XLH is characterized by altered bone mineralization due to impaired phosphorus absorption and reabsorption, as a result of mutations in the PHEX gene. The bone tissue «softens», and this process is accompanied by deformities in long tubular bones. In this article we describe the family, in which both diseases are presented, despite their rarity. The case is investigated from points of view: the clinician’s and the patient’s perspective.