Previous research on the incidental acquisition of vocabulary through reading has suggested a relationship exists between incidental acquisition and already acquired vocabulary knowledge, with higher proficiency participants incidentally acquiring more words than lower proficiency participants. Such claims have been based largely on arbitrary grouping of participants or by administering the Vocabulary Levels Test (VLT). A more conclusive result may be obtained by administering the Vocabulary Size Test (VST) rather than the VLT. The VST was developed as one solution for determining the size of an L2 learner’s vocabulary. The VST is different from the VLT in that the VST measures the current size of a learner’s vocabulary in contrast to the VLT that ought be used to determine whether L2 learners should focus on learning of specific vocabulary levels. To investigate whether a relationship exists between current vocabulary knowledge and the amount of vocabulary incidentally acquired through reading, L2 participants (n = 32) were given a copy of an unmodified English novel containing 49 target nonce words to read within two weeks. After reading, they were given two surprise forms of assessment (meaning recall translation and meaning recognition multiple-choice) measuring acquisition. A statistically significant medium correlation between the VST and meaning recall assessments was found. A statistically significant large correlation between the VST and meaning recognition assessments was also found. The stronger correlation between the VST and meaning recognition results could be due to the fact that both the VST and meaning recognition assessed learners’ receptive vocabulary knowledge (i.e., multiple choice format) whereas, the meaning recall, a translation task, required productive knowledge of the vocabulary assessed. The strong correlations between participants’ target language knowledge as shown on the VST and their performance on the incidental vocabulary acquisition measurements support previous research findings. Pedagogical and research implications, including vocabulary assessment practices, are further discussed.