Undertaken from a cognitive perspective, this study investigates how lower level processing (L2 linguistic knowledge) and higher level processing (background knowledge) influence English reading comprehension of university EFL learners in Taiwan. In particular, the study examines the bilateral effects of these two knowledge bases on the students' performance of a summary writing after reading an article in English which contained subject matter information. A TOEFL reading proficiency test accompanied by a background knowledge test developed by field experts were used to measure the two variables. Seventy-one university students who exhibited different levels of L2 reading proficiency and background knowledge participated in the study. The effects of linguistic proficiency vs background knowledge were scrutinized through statistical measures. The analysis revealed that the level of English proficiency and background knowledge both affected the participants' performances on summary writing into L1; however, the role of background knowledge, being a more powerful predictor of performance, was an integral component of comprehension in academic reading. The concomitant effects of these two variables were not observed, suggesting one knowledge base could not compensate for deficiencies in the other. In addition, the study indicated that university students' understanding of subject matters may extend beyond their own disciplinary area, as English majors might be equipped with more background knowledge than their peers majoring in that specialized discipline; likewise, non-English majors might outperform their English major counterparts in English reading proficiency test. In general, the students performed relatively poor in summary writing since they failed to build a conceptual synthesis based on the reading article. Suggestions toward improvements on English reading and summary writing are proposed for EFL university students.