This article presents a cross-cultural comparison of the size and composition of the expressive vocabulary of young children speaking two dialectal varieties of South American Spanish. Ninety-one Peruvian and 91 Argentinian toddlers (mean age: 22.5 months), matched on gender, age and maternal education, were assessed through the respective adaptations of the "MacArthur-Bates Communicative Development Inventories." Results revealed a vocabulary growth compatible with a spurt in both groups. A marginally significant country-by-gender interaction indicated that while Argentinian children exhibited an accelerated lexical growth before age 2;0 regardless of gender, only Peruvian girls did so; Peruvian boys presented a more gradual and later increase. We also found between-group qualitative differences in vocabulary composition: Argentinian toddlers with vocabularies of up to 100 words exhibited a significantly higher proportion of Nouns, while Peruvian children, especially those with a lexical mass under 50 words, had a higher percentage of Social words. Both findings are discussed in terms of possible cross-cultural differences linked to input quality and early interactions, and avenues for further research are proposed.