With the goal of fully guaranteeing the constitu- tional right to health protection, Mexico City’s left- ist administration (2000-2006) undertook a reform to provide health services to people without insur- ance. The reform had four components: free medi- cine and health services; the introduction of a new service model (MAS); the strengthening, expan- sion, and improvement of services, and legislation to ensure that the city government become guaran- tor of this constitutional right. The reform resulted in 95% of eligible families being enrolled in free care; expansion of health care infrastructure with the construction of five new health care centers and a 1/3 increase in the number of public hospital beds in impoverished and disadvantaged areas; increased access to and use of health services particularly by the poor and for expensive interventions; and the legal guarantee of the continuity of this policy. The implementation of this new policy was made possi- ble through an 80% budget increase, improvements in efficiency, and a successful fight against corrup- tion. The health impact of the reform was seen in decline of mortality rates in all age groups between 1997 and 2005 (22% for child mortality, 11% for economically active age groups, and 7.9% for retired age groups) and by a 16% decline in AIDS related mortality between 2000 and 2005. This reform contrasts with the health care reform pro- moted by the right wing Federal government in the rest of the country; the latter was based on volun- tary health insurance, cost-sharing by families, ac- cess to a limited package of services, and gradual enrollment of the population not covered by the Social Security System.