Vocabulary assessment is an important part of measuring language proficiency in both monolingual and bilingual children. The LITMUS Cross-Linguistic Lexical Tasks (CLT) provides a framework for assessing the vocabulary of monolingual and bilingual children using a standardized procedure and comparable stimuli across languages. All language versions of CLT include picture naming and picture recognition tasks, using both nouns and verbs as target words. The present study demonstrates high reliability (internal consistency) and convergent validity of the Polish CLT (CLT-PL) in a monolingual sample of children between 3 and 6 years of age. We present data collected from 479 participants. Based on the analysis of the impact of children's demographic characteristics on CLT scores, we developed a set of monolingual norms for children aged 3;0 to 5;11. We conclude that the CLT-PL is a reliable and valid tool for assessing vocabulary in monolingual children that could be applied in research and has the potential to serve as a diagnostic tool in the future. With this, we are one step further in designing tools for assessing the vocabularies of multilingual children learning Polish.