This paper examines the case of Mexican immigrants in the United States and their access to medical services within a political economy of health framework. Such an approach stresses that the provision of health care is independent of health factors per se and that access to health care is not equally distributed throughout a population. The first section reviews the three major concepts influencing medical anthropologists working within a political economy framework: the social origins of illness; the allocation of health resources; and fieldwork in Third World countries. The analysis then focuses upon the reasons for limiting immigrants' access to health care, followed by an examination of the socioeconomic characteristics of Mexican immigrants, including an undocumented immigration status, which limit their access to health care. Finally, the consequences of limited access to health services are suggested, including a low utilization of preventive health services resulting from the high cost of care combined with the immigrants' generally low income, lack of medical insurance and fear of using U.S. health services.