The emergence of grammar in relation to lexical growth was analyzed in a sample of Finnish children (N=181) at 2 ; 0. The Finnish version of the Communicative Development Inventory was used to gather information on both language domains. The onset of grammar occurred in close association with vocabulary growth. The acquisition of the nominal and verbal inflections of Finnish differed when analyzed in relation to the lexicon in which they are used: the strongest growth in the acquisition of case form types occurred when the nominal lexicon size was roughly between 50 and 250 words, whereas verb inflectional types were acquired actively from the beginning of the verb lexicon acquisition. The findings extend the previous findings of the close association between lexicon and grammar (e.g. Bates & Goodman, 1999). The results suggest that different grammatical structures display different degrees and types of lexical dependency.