This study investigated the contributions of L1 proficiency, L2 proficiency, background knowledge, and passage sight vocabulary to L2 vocabulary acquisition through reading. The relative contributions of these factors were also examined. The participants were 121 Korean middle school students studying English as a foreign language. The independent variables first included completion of the reading section of the National Academic Achievement Test of Korean and English, a background knowledge questionnaire, and a passage sight vocabulary test. The participants read a passage and subsequently took a vocabulary posttest. Multiple regression analysis revealed that L1 proficiency was not a significant predictor of L2 vocabulary acquisition, suggesting that L1-L2 interdependence is somewhat limited insofar as incidental vocabulary acquisition. Also, the results revealed that passage sight vocabulary had the largest significant effect, L2 proficiency had a large significant effect, and background knowledge had a moderate significant effect on L2 lexical development through reading. The implications of the findings in relation to L2 vocabulary instruction were also discussed.