The transmissible brain diseases of humans and animals, the spongiform encephalopathies, continue to stimulate interest, and the announcement that exposure to "mad cow disease" (bovine spongiform encephalopathy [BSE]) is a possible explanation for more than 10 cases of a variant Creutzfeldt-Jakob disease in humans in the United Kingdom is a recent example. Cases of iatrogenic Creutzfeldt-Jakob disease (from previous use of human cadaveric tissues for pituitary hormone therapy and neurosurgical grafts) are still being identified, and the unique nosological status of this group of disorders-that they are both transmissible and inherited and that the only known component of their infectious agent is protein-alone makes these diseases remarkable.