Systematic search for Creutzfeldt-Jakob disease (CJD) in France yielded a total of 170 patients dying between 1968 and 1977. The overall annual mortality rate from CJD for France was 0.32 case per million persons, with by far the highest rate (1.33 cases per million) in the most densely populated parts of Paris. A highly significant correlation between population density and the mortality rate of CJD in the Paris urban agglomeration was consistent with the hypothesis of human-to human disease transmission, although in the rest of France CJD occurred at equally low rates among rural and urban populations. Temporospatial case clustering was not observed, nor could occupation, surgery, or potential exposure to scrapie be associated with an increased risk of contracting CJD. Six to 9% of cases were familial, analysis of which discouraged the notion of a familial "common exposure" in favor of genetic transmission of susceptibility to the virus or of the virus itself. A prospective epidemiological study or newly occurring cases of CJD, especially familial and geographically isolated cases, is being undertaken in the hope of identifying the means by which CJD is naturally acquired.