This study reports on investigating the difference between Iranian high school students' performance on Academic Word (AW) and Daily-Life Word (DLW) tests. To this end, a number of 120 male senior high school students were randomly selected from twelve high schools in Gachsaran to serve as the participants of the study. Two multiple-choice tests, each comprising 40 items, were administered to the participants in order to gauge their knowledge of AWs and DLWs. Then, using purposive sampling, fifteen high school English teachers were interviewed to explore their opinions about the treatment of DLWs in high school English textbooks. To analyze the data, a paired-samples t-test was carried out to compare the participants' gains on both tests and the interviews were transcribed to pinpoint important themes. Results of the t-test indicated that the participants performed significantly better on AW test than on DLW test. In addition, results of the interviews boiled down to the fact that English materials developed for Iranian high school students severely lack an acceptable coverage of DLWs.