This paper reports a set of Monte Carlo simulations designed to evaluate the main claims made by Laufer and Nation about the Lexical Frequency Profile (LFP). Laufer and Nation claim that the LFP is a sensitive and reliable tool for assessing productive vocabulary in L2 speakers, and they suggest it might have a serious role to play in diagnostic evaluations of learners. The simulations suggest that LFP is not in fact all that sensitive. It works best when the groups being compared have very disparate vocabulary sizes, and is probably not sensitive enough to pick up modest changes in vocabulary size.