This study explored the specific effects of vocabulary knowledge, transferred from a first language, in the development of second language reading proficiency. The study was conducted with 24 college students enrolled in a first-year Japanese program. Specially, the study examined the effects of transferred vocabulary knowledge on: (a) the acquisition of L2 linguistic knowledge, (b) verbal processing skills, and (c) reading comprehension. It was found that: (1) L2 vocabulary knowledge was most highly correlated with reading comprehension; (2) that vocabulary knowledge was the single most signifcant factor differentiating students with related L1 orthographic backgrounds from those with unrelated L1 orthographic backgrounds, and (3) that differences in test performance between the two groups became significantly greater over time. These findings seem to suggest that transferred vocabulary knowledge increases L2 reading Comprehension; moreover, the initial advantage magnifies its effects over time as task complexity increases, thus enhancing the overall development of L2 reading proficiency.