Recent studies such as the Commission on Macroeconomics and Health have highlighted the need for expanding the coverage of services for HIV/AIDS, malaria, tuberculosis, immunisations and other diseases. In order for policy makers to plan for these changes, they need to analyse the change in costs when interventions are ‘scaled-up’ to cover greater percentages of the population. Previous studies suggest that applying current unit costs to an entire population can misconstrue the true costs of an intervention. This study presents the methodology used in WHO-CHOICE's generalised cost effectiveness analysis, which includes non-linear cost functions for health centres, transportation and supervision costs, as well as the presence of fixed costs of establishing a health infrastructure. Results show changing marginal costs as predicted by economic theory. Copyright © 2004 John Wiley & Sons, Ltd.