Sixty English as a foreign language learners were divided into high-, intermediate-, and low-level groups based on their scores on pretests of target vocabulary and Vocabulary Levels Test scores. The participants read 10 Level 1 and 10 Level 2 graded readers over 37 weeks during two terms. Two sets of 100 target words were chosen from each set of graded readers and were tested on three occasions. The results showed that the relative gains from pretest to immediate posttest were 63.18%, 44.64%, and 28.12% for the high-, intermediate-, and low-level groups, respectively. There was little decay in knowledge on the Term 1 three-month delayed posttest; relative gains ranged from 21.05% for the low-level group to 59.01% for the high-level group. The learning gains in Term 2 were consistent with those from Term 1. The results indicate that prior vocabulary knowledge may have a large impact on the amount of vocabulary learning made through extensive reading.