From an early age, children show a tendency to map novel labels onto unfamiliar rather than familiar kinds of objects. Accounts of this tendency have not addressed whether children develop a metacognitive representation of what they are doing. In 3 experiments (each N = 48), preschoolers received a test of the "metacognitive disambiguation effect," which involved deciding whether the referent of a novel label was located in a bucket of things "I know" or bucket of things "I don't know." Most 4-year-olds passed this test, whereas most 3-year-olds did not. Children's performance was predicted by their ability to report whether various words and pseudowords were ones that they knew, even after age and vocabulary size were controlled. As children develop an awareness of their lexical knowledge/ignorance, they also develop a metacognitive representation of their tendency to map novel labels onto unfamiliar rather than familiar kinds.