Two-thirds of osteogenesis imperfecta (OI) patients claimed that they bruised easily. We have studied haemostasis in 58 subjects with OI, tarda type. The most frequent abnormalities were increased capillary fragility (35%), decreased platelet retention (33%) and reduced factor VIII R:Ag (23%). Reduced ristocetin cofactor, deficient platelet aggregation induced by collagen and prolonged bleeding time were less common findings. The combination of vascular, platelet related and plasmatic defects may reflect that OI is a heterogeneous group of disorders with common clinical expression. The tourniquet test is the most valuable screening test of the haemostatic defects observed in OI.