Child Health Days (CHDs) are twice-annual campaign-style events designed to increase the coverage of vitamin A and one or more other child health services. Although more than two dozen countries have had a CHD, little has been published about them. This paper presents an activity-based costing study of Ethiopia's version of CHDs, the Enhanced Outreach Strategy (EOS). The December 2006 round reached more than 10 million beneficiaries at an average cost per beneficiary of US$0.56. When measles is added, the cost of the package doubles. Given the way the distribution day delivery system and the service package are structured, there are economies of scope. Because most of the costs are determined by the number of delivery sites and are independent of the number of beneficiaries, other things equal, increasing the beneficiaries would reduce the average cost per beneficiary. Taking into account only the mortality impact of vitamin A, EOS saved 20,200 lives and averted 230,000 DALYs of children 6-59 months. The average cost per life saved was US$228 and the cost per DALY averted was equivalent to 6% of per capita GDP (US$9), making the EOS cost-effective, according to WHO criteria. While CHDs are generally construed as a temporary strategy for improving coverage of supply-constrained systems, inadequate attention has been paid to demand-side considerations that suggest CHDs have an important role to play in changing care-seeking behaviour, in increasing community organization and participation, and in promoting district autonomy and capacity. Recognition of these effects suggests the need for decisions about where and when to introduce, and when to end, a CHD to take into account more than 'just' health sector considerations: they are more broadly about community development. UNICEF played a key role in initiating the EOS and finances 68% of costs, raising concern about the programme's long-term sustainability.