Mortality from childhood leukemia was examined particularly in rural countries in relation to any major rural-urban migration. Significant increases have been found in other situations of rural population mixing as predicted by the infection hypothesis. The 1950s and 1960s were of most interest since it preceded the decline in mortality brought about by effective chemotherapy in many countries. The 33 countries covered were all those in the World Health Organization's mortality database. No sensitive measure of rural-urban migration is available for international comparisons. However, it seems noteworthy that Greece and Italy, the two countries with the most striking levels of rural migration in the 1950s and 1960s, also had unusually high mortality rates from childhood leukemia. Greece was most affected proportionally by these population movements and from 1958 to 1972 had the highest recorded mortality from this cause in the world. The problems of international comparisons of mortality data dictate caution in drawing conclusions. However, against a background of other work on population mixing, and in the light of certain considerations, we suggest that the marked rural population mixing in Greece and Italy may have contributed to their high mortality rates from childhood leukemia in the 1950s and 1960s.