This study was an attempt to investigate a Cognitive Academic Language Proficiency (CALP) theme with the Interdependence Hypothesis in the use of Content-Based Instruction (CBI) approach. In addition, the study examined the threshold notion of language proficiency and how it would play in different groups distinguished by school level. Based on the analyses of data obtained from comprehension quizzes, proficiency tests of speaking and writing, questionnaire, and interview, the study revealed that the undergraduate group showing a higher level of proficiency and the graduate group experiencing longer period of schooling performed on the comprehension quizzes and evaluated their CBI experiences similarly. It suggested that CALP represented by schooling earned in L1 be taken advantage of as a springboard for CBI. It also indicated that reading skills may be more critical to the survival and success in CBI courses at least for the participants of the current study who showed relatively high confidence in listening skills. It suggested that there is a need for further study to examine whether such a tendency is common to the similar groups of Korean English language learners in order to accommodate their preferred way of learning English.