This paper develops a scalar or quantitative measure of the “distance” between English anda myriad of other (non-native American) languages. This measure is based on the difficultyAmericans have learning other languages. The linguistic distance measure is then used in ananalysis of the determinants of English language proficiency among adult immigrants in theUnited States and Canada. It is shown that, when other determinants of English languageproficiency are the same, the greater the measure of linguistic distance, the poorer is therespondent’s English language proficiency. This measure can be used in research,evaluation and practitioner analyses, and for diagnostic purposes regarding linguisticminorities in English-speaking countries. The methodology can also be applied to developlinguistic distance measures for other languages.