Hwangbo, Young-Shik. 2017. [u]-reduction and [w]-epenthesis in American English. Korean Journal of English Language and Linguistics 17-2, 351-378. The purpose of this paper is to examine the conditions for the reduction of unstressed prevocalic [u] and the [w]-epenthesis in American English, which are observed in such words as tenuous [ˈtɛ nju əs] ~ [ˈtɛ njə wəs], based on the The Carnegie Mellon University Pronouncing Dictionary (CMUdict). The words containing unstressed prevocalic [u] were collected from CMUdict and classified into three groups: words with [w]-epenthesis, words without [w]-epenthesis, and words fluctuating between the two. The relationship between [w]-epenthesis and environments (stress patterns surrounding [u] and the type of consonants before [u]) has been analyzed. In addition, the relationship between [w]-epenthesis and the frequency of use of words has been analyzed. The results showed that the rate of [w]-epenthesis cannot be explained by any one of the conditions. It was revealed, instead, that stress patterns surrounding the [u], consonantal environment, and high frequency of use all work together to raise the rate of [w]-epenthesis. It should be noted, however, that the frequency of use has strong effects on the rate of [w]-epenthesis, but only when some conditions were met, elsewhere it may not have much influence on the rate of [w]-epenthesis.