In child abuse cases involving multiple fractures, prosecutors and investigators are increasingly facing a relatively new defense. In some jurisdictions, judges are allowing defense medical experts to testify that infants have not been abused, but instead suffer from a mild form of Osteogenesis Imperfecta (OI) or a purported variant of OI, Temporary Brittle Bone Disease (TBBD). These diagnoses are offered in cases where the injuries are highly specific for abuse because they involve: (1) fractures typical of abuse in different stages of healing; (2) infants who have tested negative for conventionally diagnosable metabolic bone diseases (including OI); and (3) infants whose bones do not continue to fracture after they are placed in protective custody. 2 It is critically important for doctors, investigators and prosecutors to be able to distinguish bone disease from abuse because OI is the most frequent medical/legal defense in suspected cases of child abuse.3 This article will provide a brief and general overview of what is currently known and accepted in the medical literature about OI, and then examine more controversial diagnoses such as TBBD. Finally, strategies for prosecutors will be discussed for dealing with bone disease defenses.