International comparisons of average annual age-adjusted mortality rates for primary tumours of the nervous system for 1951 through 1958 and 1967 through 1973 showed marked geographical variation for both study periods. For the majority of countries, the mortality rates increased by at least 40% in the in the intervening 15-year period, while in 20% of the countries the rates increased by over 100%. The percentage increases varied from 12.2 to 345.5. The improvement in the diagnosis of these tumours, particularly among elderly individuals (who have the highest age-specific incidence rates for these neoplasms), presumably accounts for most of this change. However, it is possible that the increase may not be fully accounted for by these factors alone. It is therefore suggested that further epidemiological studies are undertaken to determine more accurately the causes and implications of this worldwide increase in mortality rates from primary tumours of the nervous system.