This paper reports on the results of an intensive study of the costs and cost-effectiveness of the Expanded Programme on Immunization (EPI) in The Gambia. Costs in 1988, for the nation as a whole and for a large sample of specific local delivery sites, and their relation to vaccination coverage are presented. The total and average costs of the full national EPI fell impressively in The Gambia from about US$1.50 per dose in the early 1980s to US$0.60 in 1988. That conclusion is reinforced by improvements in vaccination coverage and by a very large decline in cost per fully immunized child from about US$19.00 to US$5.64. Comparisons of the latter finding with the earlier Gambian value, and with other developing countries' figures, suggest a highly cost-effective EPI for 1988. Some of the major reasons for this were good administration, widespread deployment of delivery teams, and effective social mobilization (community awareness) efforts. The study offers site-specific cost estimates that reveal large variations in average costs among centres. Lower average costs appear to be based on economies of scale and greater intensity of use of facilities and teams, together with better management. As with the estimates for the full national programme, these variations are not explicable simply as artefacts of methodological techniques and assumptions. Although recent (1988) Gambian data indicate relatively low costs and high cost-effectiveness, the continuation of, and probable increase in, other threats to health than vaccine-preventable diseases pose daunting challenges to all parties concerned with improving the allocation of scarce resources for the wellbeing of the Gambian people, and of others in the developing world.