Efforts to address the health care problems of undocumented migrants and refugees have typically lacked reliable information about their health status over time, their use of health services, and the nature and range of barriers that limit their access to adequate health care. This is the first comparative study of two sizable populations: Mexican immigrants (including both undocumented and legal permanent residents) and Southeast Asian refugees (from Vietnam, Cambodia and Laos, including the Hmong from Laos and ethnic Chinese from Vietnam). These populations represent polar opposite types of migrations, with many cultural, psychosocial and historical differences in both their contexts of exit and of reception in the Unites States. Structurally, however, these populations share many problems that limit their present and future access to health care. Their economic and legal-political status significantly affect their search for and utilization of health services. Both groups must also confront a variety of problems that arise out of cultural differences between medical practitioners and patients. A comparative analysis of their predicament sheds light on the nature and politics of migrant health care, and on the attendant dilemmas for health care planning and policy. The study is based on comprehensive surveys of large samples of Mexican immigrants and Indochinese refugees; nearly 3,000 in-depth interviews conducted in over 1,500 households in San Diego County, California; and field research in area hospitals and clinics. It examines the migration histories and social backgrounds of these populations, their demographic profiles, the range of health problems found among them, and factors affecting their health status and access to health care services — including legal, economic, social and cultural barriers that define their experience with the health care system — which are detailed both quantitatively and via qualitative case histories. The paper concludes with a review of policy options for improving access to health care for these populations.