Osteogenesis imperfecta (OI) is a heterogeneous con­nective tissue disorder characterized primarily by fragile, easily broken bones, but also by blue sclerae, dental ab­normalities (dentinogenesis imperfecta), thin skin, weak tendons, and progressive hearing loss [1-3]. Severity ranges from death in the perinatal period to normal life-span with slightly increased propensity towards bone frac­tures. Four types of OI are classified on the basis of pheno­typic severity [2,4]. In the most severe form (type II), death occurs in utero or shortly after birth. Types III and IV are moderate-to-severe variants of the disease, type III being the more severe and characterized by progressive deformity of long bones, spine and skull. Type I, the mild­est form, presents with blue sclerae and mild bone defor­mities, causing short stature. Most cases are caused by dominant mutations in COL1A1 or COL1A2, the genes encoding the a1(I) and the a2(I) subunits of type I colla­gen, respectively [5,6]. Defects in COL1A1 that result in the synthesis of half the normal amount of collagen cause OI type I. Severe types are caused by structural defects in either the a1(I) or a2(I) chains.

Therapy with bisphosphonates has proved successful in children with OI [7-11]. However, there are no follow-up data on the amount of collagen produced by cultured fibroblasts of patients during treatment. Electrophoretic and quantitative analyses of collagen in fibroblasts, cul­tured from skin biopsies taken during treatment of two patients with type I OI, are presented together with the results of treatment.