The main aim of this study, which presents the Slovenian adaptation of the Macarthur-Bates Communicative Development Inventory CDI–III, was to investigate the characteristics of language development in monolingual Slovenian-speaking children aged 30–48 months. In addition, we examined the relationships between different measures of child language assessed by the CDI–III, namely vocabulary, grammar and metalanguage. The sample comprised 301 children whose language was assessed by their parents using the Slovenian version of the CDI–III. The results indicate that language development at this age continues to progress relatively quickly, particularly in terms of children’s metalinguistic abilities, although there are large individual differences in language ability between children of the same age. The findings also indicate that some of the pre-existing relationships established between the different domains of infant and toddler language ability persist into early childhood, with vocabulary emerging as an important predictor of children’s grammar.