Objectives: Social workers will increasingly be required to attend to the cost-effectiveness of practices, programs, and policies. In the area of substance abuse, there is little evidence to suggest that social workers' decisions are based on evidence of either effectiveness or costs. Method: This article provides an overview of existing evidence on the effectiveness, costs, and benefits of methadone maintenance in the treatment of opiate addiction. Results: Methadone maintenance is a demonstrably cost-effective treatment for heroin dependence. Conclusions: As yet, social workers do not appear to be aware of either the effectiveness or the cost advantages of methadone. Because findings supporting the cost-effectiveness of methadone are stronger than in most areas of social work practice, the extent to which social workers appropriately refer and advocate for clients who can benefit from this treatment can be seen as a bellwether of how the profession will perform in the emerging era of cost-effectiveness.