It has been shown previously that in many developing countries the reported incidence of childhood cancer for boys is substantially higher than for girls and that this difference is related to economic development of the country under study. The aim of the present study was to investigate the relationship between the sex ratio of children registered with malignant diseases and national economic development. Sex ratios for childhood cancer registration were obtained for populations within 53 countries from the second volume of the International Incidence of Childhood Cancer (range boys:girls 1.09-2.05). Demographic, economic and health data for the corresponding countries were obtained from published statistics and used to determine the extent to which sex ratios were associated with national economic status. The ratio of boys:girls registered with childhood cancer increased with decreasing gross domestic product and with increasing infant mortality, suggesting that boys are increasingly more likely than girls to be registered with increasing economic disadvantage. Combining these results with childhood cancer registration data from northern England suggests that little of the predominance of boys reaching a specialist centre can be explained by differences between boys and girls in early deaths from the disease. The previously observed differences in the childhood cancer registration sex ratio in developing countries still exist. Hence, international differences in the incidence of childhood cancer should be interpreted cautiously as they may not necessarily reflect only differences in the underlying occurrence of disease