This paper examines survey data gathered from 2103 Mexican immigrants living or working in San Diego County, California, in order to explore four fundamental questions concerning the utilization of health services: (a) What type of health services do Mexican immigrants use? (b) When hospitals are used, do they tend to be emergency room services? (c) Do Mexican immigrants use preventive services? (d) To what extent do the utilization patterns of undocumented immigrants differ from their legally-immigrated counterparts? The socioeconomic profile of the sample is characterized through analysis of variables such as sex, age, length of residence in the U.S., occupation and income. Mexican immigrants, particularly the undocumented, are relatively young compared to the non-immigrant population, of short duration in the U.S. and earn low income. In addition, undocumented and legally-immigrated respondents are covered by medical insurance at rates far below the general population. Mexican immigrants, including the undocumented, use a variety of health services. Hospital services are not the primary source of care. However, when undocumented respondents did use hospital services, they were more likely to use emergency room care than their legally-immigrated counterparts, who were more likely to use out-patient services. Finally, undocumented respondents tended to neglect preventive services as evidenced by examination of the use of pre-natal care, general check-ups and dental services.