The worldwide epidemiology of Creutzfeldt-Jakob disease (CJD) is presented from an analysis of 1,435 patients. In the United States, the average annual mortality rate is at least 0.26 deaths per million. Temporal-spatial clustering of cases was not found in the United States, but reports from other countries indicate that such clustering does occur. Fifteen percent of the cases were of the familial type, suggesting a genetic susceptibility to infection. Iatrogenic transmission by corneal transplantation and neurosurgical operations has occurred, and the possibility is raised that previous surgery or preexisting neurological operations has occurred, and the possibility is raised that previous surgery or preexisting neurological disease may be associated with an increased risk of developing CJD. It remains to be determined whether the virus of CJD is maintained only by patient-to-patient transmission, has a zoonotic reservoir such as scrapie, or causes widespread latent infection of man that is occasionally activated.