Lexical use seems to represent one of the major problems Korean learners face in learning English. One of the factors attributable to the learners' lexical problems may be in large part due to the use of the Korean language as a medium of instruction in teaching English. Fifty native Korean students took part in this research. They were of 20 male and 30 female students majoring in different subjects at a university in Korea. This study is intended to address some aspects of lexical use in interlanguage of Korean English as a Foreign Language (EFL) learners. The findings of this study revealed that most of the lexical deviations were traceable to both problems with the target language itself (intralingual) and transfer effects from the native language (interlingual). The students' problems with aspects of the target language itself constituted a high percentage (62.4%) of the total number of deviant lexical uses. It is not surprising that the students who have limited experience in English may face difficulties expanding and using vocabulary appropriately in different contexts. In order to overcome these difficulties, they might have resorted to different strategies. However, because of their insufficient and imperfect knowledge of English, the students produced deviant lexical items. The results of this study yielded some implications for vocabulary pedagogy in the EFL context. Although the high occurrence of the problematic uses might not necessarily mean that they have great difficulties with those areas, the potential sources of problems must be given special consideration in language instruction. (Contains 1 table.)