In Chile, the National Depression Detection and Treatment Program (Programa Nacional de Diagnóstico y Tratamiento de la Depresión, PNDTD) in primary care is a rare example of an evidence-based mental health program that was scaled up to the national level in a low- or middle-income country. This retrospective qualitative study aimed to better understand how policymakers made the decision to scale up mental health services to the national level, and to explore the elements, contexts, and processes that facilitated the decision to implement and sustain PNDTD. In-depth semistructured interviews with six key informants selected through intentional sampling were conducted in August-December 2008. Interviewees were senior officers at the Ministry of Health who were directly involved in the decision to scale up the program. Results yielded four elements pivotal to the decisionmaking process: scientific evidence, teamwork and leadership, strategic alliances, and program institutionalization. Each element contributed to building consensus, securing funding, attracting resources, and gaining lasting support from policymakers. Additionally, a review of available documentation led the authors to consider sociopolitical context and use of the media to be important factors. While research evidence for the effectiveness of mental health services in the primary care setting continues to accumulate, low- and middle-income countries should get started on the lengthy process of scaling up by incorporating the elements that led to decisionmaking and implementation of the PNDTD in Chile.