One crucial aspect of CLIL-based foreign language learning in instructional settings is vocabulary growth.As a consequence, research should be interested in how CLIL fosters vocabulary learning.Noticing an apparent shortage of data-driven quantitative research on vocabulary growth in this field of CLIL is, therefore, problematic.The present paper reports findings from a mixed-methods study of vocabulary growth in an Austrian lower secondary school CLIL setting, with English as the language of instruction and learning.The aim of the study was to analyse how the use of CLIL in the English classroom could benefit learners in their acquisition of vocabulary in the target language.First, a repeated-measure-design with experimental and control groups assessed receptive vocabulary growth by means of a standardized vocabulary size test.Second, students' questionnaire data as well as vocabulary profiling of the CLIL teachers' linguistic input explored possible covariates for the vocabulary test scores.We found that CLIL-related effects were only co-determined by input frequency, while extra-mural factors did not play any role in this study.As a consequence, overly optimistic expectations regarding the linguistic impact of CLIL in a mixed-ability setting guided by a predominantly implicit language teaching approach need to be re-evaluated critically.