Vocabulary size measures serve important functions, not only with respect to placing learners at appropriate levels on language courses but also with a view to examining the progress of learners. One of the widely reported formats suitable for these purposes is the Yes/No vocabulary test. The primary aim of this study was to introduce and provide preliminary validity evidence for a lemma-based vocabulary size test (LVST), which has been designed to measure L2 learners' receptive vocabulary size of English. The test was administered to 219 participants. Various validity measures were operationalized to examine the performance of the test. Results show high reliability indices for the test in terms of parallel forms and internal consistency. The results also indicate that a single dimension underlies the test construct, and that the test is capable of distinguishing learners of varying proficiency levels. Concerning the item difficulty, results show approximately a linear order of difficulty across frequency levels. The test also correlated significantly with a measure of general English proficiency. The results, collectively, suggest that the LVST can usefully measure the intended construct of receptive vocabulary size, and greatly extends the range of vocabulary size measurement provided by other Yes/No tests.