Previous work suggests that phonological neighborhood density is a key factor in shaping early lexical acquisition. Such studies have, however, have not considered how semantic neighborhoods may influence word-learning. We studied how phonological and semantic densities affect both comprehension and production of nouns from the Macarthur-Bates Communicative Development Inventory (MCDI). New measures of semantic and phonological densities, along with child-directed word frequency counts were used to predict the percentage of children who know each word at different ages (8 - 30 months) as indicated in MCDI lexical norms. Production was predicted by frequency and phonological density at all time points, replicating previous research. Semantic density predicted production only at 30 months. Comprehension norms were predicted by frequency and semantic density, and never by phonological density. Two- and three-way interactions reveal that semantic density may moderate effects in production, while sound density may moderate effects in comprehension.