Prior research indicates that young children can generalize object properties on the basis of category information when it is conveyed by identical labels or semantically similar labels at the same level of taxonomy (i.e., synonyms) (Gelman & Markman, 1986). However, in previous research semantic similarity was confounded with co-occurrence probability. Therefore, it is possible that synonym-based induction observed in prior research stemmed from children relying on cues other than the semantic similarity of labels. The present study investigated synonym-based induction with labels that do and do not co-occur in child-directed speech. Results indicated that adults made inferences on the basis of the semantic similarity of labels regardless of co-occurrence probability. In contrast, 4-year-old children generalized based on synonymous labels at above chance levels only when synonyms co-occurred in child-directed speech.