OBJECTIVES: Identify the make-up of the Spanish-speaking population in the United States; assess the impact of immigration on language; analyse issues faced during linguistic validation of Clinical Outcomes Assessments (COA). METHODS: We investigated population numbers of native-Spanish speakers in the US; researched statistics regarding country-of-origin; reviewed linguistic validation projects for the U.S. and Latin America (LATAM); compared the results. RESULTS: The 2010 census revealed that 16% of the U.S. population were of Hispanic or Latino origin, with 13% of children under five years old speaking a form of Spanish at home. We discovered that the distribution of Spanish speakers varies according to country of origin, with pockets of high densities. For example, regionally, the South is home to the majority of South American Hispanics (42%), the West is home to 51% of Mexicans, and the Northeast has welcomed over three quarters of Dominican Republicans in the U.S.. Hispanics of Mexican origin have the highest population of Spanish-speakers in the U.S. but those of South American and other Central American origins have markedly increased since 2000. There are significant differences in terminology used across Spanish-speaking populations. One example of this variation is the use of 'once every other day', a common instruction for COA or medication administration. The various translations used LATAM included 'one day yes and one day no', 'take once a day on alternating days' and 'every third day'. This diversity demonstrates the number of influences on the U.S. Spanish language due to the varied population, but is only the tip of the iceberg. CONCLUSIONS: We recommend that, during linguistic validation, a specific translation is developed for use in the U.S. by working with linguists from a range of LATAM countries. This will ensure a neutral translation is created that will be acceptable to the diverse range of Spanish-speaking individuals throughout the U.S.