This study explores the roles of depth and breadth of lexical repertoire in L2 lexical inferencing success and incidental vocabulary acquisition through reading. Students read a graded reader containing 13 pseudo-words and attempted to infer the meanings of underlined target words. The Word Associates Test (WAT, Read, 2004) and the Vocabulary Levels Test (Schmitt, Schmitt, & Clapham, 2001) were administered to measure depth and breadth of lexical repertoire respectively. To rate retention of inferred meanings, I administered the Vocabulary Knowledge Scale (VKS, Paribakht & Wesche, 1996, 1997) with a repeated measure design. The results indicated that (a) both breadth and depth of lexical knowledge correlated positively with long-term retention of inferred word meanings. However, depth of vocabulary knowledge indicated a higher correlation; and (b) scores on both breadth and depth of vocabulary knowledge had a significant positive correlation with success of lexical inferencing through reading, but depth of vocabulary knowledge was a stronger predictor of inferencing success. (Contains 4 tables and 1 figure.)