Background BCG vaccine has shown consistently high efficacy against childhood tuberculous meningitis and miliary tuberculosis, but variable efficacy against adult pulmonary tuberculosis and other mycobacterial diseases. We assess and compare the costs and effects of BCG as an intervention against severe childhood tuberculosis in different regions of the world. Methods We calculated the number of tuberculous meningitis and miliary tuberculosis cases that have been and will be prevented in all children born in 2002, by combining estimates of the annual risk of tuberculosis infection, the proportion of infections that lead to either of these diseases in unvaccinated children, the number of children vaccinated, and BCG efficacy. Findings We estimated that the 100· 5 million BCG vaccinations given to infants in 2002 will have prevented 29 729 cases of tuberculous meningitis (5th–95th centiles, 24 063–36 192) in children during their first 5 years of life, or one case for every 3435 vaccinations (2771–4177), and 11 486 cases of miliary tuberculosis (7304–16 280), or one case for every 9314 vaccinations (6172–13 729). The numbers of cases prevented would be highest in South East Asia (46%), sub-Saharan Africa (27%), the western Pacific region (15%), and where the risk of tuberculosis infection and vaccine coverage are also highest. At US $2–3 per dose, BCG vaccination costs US $206 (150–272) per year of healthy life gained. Interpretation BCG vaccination is a highly cost-effective intervention against severe childhood tuberculosis; it should be retained in high-incidence countries as a strategy to supplement the chemotherapy of active tuberculosis.