Nonhuman primates were inoculated intracerebrally with brain tissue from 52 patients with confirmed Alzheimer disease (AD) in order to investigate the possibility of an infectious etiology. Animals inoculated with brain tissue from two patients with familial AD developed a spongiform encephalopathy that was indistinguishable from Creutzfeldt-Jakob disease (CJD). Seventeen other cases of AD on test for more than 50 months failed to produce similar changes, and 33 cases have not been incubating for a sufficient period of time to ascertain the presence of a transmissible agent. The initial transmission of spongiform encephalopathy with brain tissue from the two familial cases of AD has not been reproduced and the association between AD and an infectious agent has not yet been demonstrated with any reasonable degree of certainty. The frequent overlap of clinical symptoms of AD and CJD, and the occurrence of cases of CJD and AD in the same families indicate the need for continuing research on the relationship between the two diseases.