This paper examines the lexical entrainment of real users in the Let's Go spoken dialog system. First it presents a study of the presence of entrainment in a year of human-transcribed dialogs, by using a linear regression model, and concludes that users adapt their vocabulary to the system's. This is followed by a study of the effect of changing the system vocabulary on the distribution of words used by the callers. The latter analysis provides strong evidence for the presence of lexical entrainment between users and spoken dialog systems.