Purpose: The purpose was to determine whether semantic set size, a measure of the number of semantic neighbors, influenced word learning, and whether the influence of semantic set size was broad, showing effects on multiple measures both during and after learning. Method: Thirty-six preschool children were exposed to 10 nonobjects, varying in semantic set size, paired with 10 nonwords, controlling phonotactic probability and neighborhood density. Nonobject-nonword pairs were presented in a game format. Learning was measured in naming and referent identification tasks administered before, during, and 1 week after training. Results: Results showed no differences in naming or identifying the referents of the nonobject-nonword pairs with small versus large semantic set sizes before and during training. However, 1 week after training, children named and identified the referents of nonobject-nonword pairs with small set sizes more accurately than those with large set sizes. Conclusions: Similarity to known representations appears to influence word learning, regardless of whether the similarity involves lexical or semantic representations. However, the direction of the effect of similarity to known representations on word learning varies depending on the specific type of representation involved. Specifically, lexical similarity speeds learning, whereas semantic similarity slows learning.