The Cross-linguistic Lexical Task (CLT; Haman, Luniewska & Pomiechowska, 2015) is a vocabulary task designed to enable cross-linguistic comparisons both across and within (bilingual) children. In this paper we assessed the validity of the CLT as a measure of language proficiency in bilingual children, by determining the extent to which: (1) age-matched, monolingual Spanish-speaking and Dutch-speaking children obtained similar scores; (2) the CLT correlated with other measures of language proficiency in monolingual and bilingual children; and (3) whether the factors underlying the CLT's construction, i.e., target words' estimated Age of Acquisition and Complexity Index, were predictive of children's scores. Our results showed that, while the CLT correlated with other measures and is therefore a valid means of tapping into language proficiency, caution is required when using it to compare children's language proficiency cross-linguistically, as scores for Dutch-speaking and Spanish-speaking monolinguals sometimes differed.