Because of its potential importance for word learning, children's judgment of whether they know names for objects was investigated. In Study 1, judgment accuracy was at or near ceiling in about two-thirds of 4-year-olds, and covaried with judgment of word familiarity and with justifying novel name mapping in terms of avoidance of name overlap. The two judgments and the justification were unrelated to general intelligence, which is evidence that awareness of lexical ignorance is a specific metalinguistic ability in this age group. Preexposure to an unfamiliar object increased the likelihood of it being misjudged as having a known name, but only by the children who scored above the median in word familiarity judgment. Thus, judgment of lexical ignorance may undergo a developmental change from relying solely on whether target information is retrieved to considering additional information. Consistent with this hypothesis, preexposure did not affect 2-and-1/2-year-olds' judgments of object name knowledge in Study 2. About one-third in this age group identified more familiar than unfamiliar objects as having known names. These judgments covaried with word familiarity judgments, but were not related to novel name mapping justifications and were related to general intelligence. Thus, awareness of lexical ignorance does not become a unified metalinguistic ability until after 2-and-1/2 years.