Our understanding of the relationship between immigration and mental health can be advanced by comparing immigrants pre- and post-immigration with residents of the immigrants' home countries. DSM-IV anxiety and mood disorders were assessed using identical methods in representative samples of English-speaking Mexican immigrants to the US, a subsample of the US National Comorbidity Survey Replication (NCSR), and Mexicans, the Mexican National Comorbidity Survey (MNCS). Retrospective reports of age of onset of disorders and, in the immigrant sample, age of immigration were analyzed to study the associations of pre-existing mental disorders with immigration and of immigration with the subsequent onset and persistence of mental disorders. Pre-existing anxiety disorders predicted immigration (OR=3.0; 95% CI 1.2-7.4). Immigration predicted subsequent onset of anxiety (OR=1.9; 95% CI 0.9-3.9) and mood (OR=2.3; 95% CI 1.3-4.0) disorders and persistence of anxiety (OR=3.7 95% CI 1.2-11.2) disorders. The results are inconsistent with the "healthy immigrant" hypothesis (that mentally healthy people immigrate) and partly consistent with the "acculturation stress" hypothesis (i.e., that stresses of living in a foreign culture promote mental disorder). Replication and extension of these results in a larger bi-national sample using a single field staff are needed.