Studies using the English and Spanish MacArthur Communicative Development Inventories demonstrated that the grammatical abilities of 20–30-month-old bilingual children were related more strongly to same-language vocabulary development than to broader lexical-conceptual development or maturation. First, proportions of different word types in each language varied with same-language vocabulary size. Second, individual changes in predicate and closed class word proportion scores were linked to growth in same-language vocabulary but not to total conceptual vocabulary. Third, increases in English utterance length and English and Spanish sentence complexity were related to growth in same-language vocabulary but not to growth in conceptual vocabulary. Increases in Spanish utterance length were linked to growth in both Spanish vocabulary and conceptual vocabulary. Possible mechanisms underlying these patterns are considered.