There are two approaches to reducing the burden of sickness and death associated with the human immunodeficiency virus (HIV), which leads to acquired immunodeficiency syndrome (AIDS): treatment and prevention. Despite large international aid flows for HIV/AIDS, the needs for prevention and treatment in low- and middle-income countries outstrip the resources available. Thus, it becomes necessary to set priorities. With limited resources, should the focus of efforts to combat HIV/AIDS be on prevention or treatment? I discuss the range of prevention and treatment alternatives and examine their cost effectiveness. I consider various arguments that have been raised against the use of cost-effectiveness analysis in setting public policy priorities for the response to HIV/AIDS in developing countries. I conclude that promoting AIDS treatment using antiretrovirals in resource-constrained countries comes at a huge cost in terms of avoidable deaths that could be prevented through interventions that would substantially lower the scale of the epidemic.