The present paper presents a comparative study of the lexical profiles of young content and language integrated learning (CLIL) and traditional English as a Foreign Language (EFL) learners' written production. The different nature and amount of foreign language input received in these classes may have consequences in learners' lexical profiles in writing. We scrutinized the writings of 72 CLIL learners and 68 traditional EFL learners for frequency bands of words used, word origin, L1 influence in lexical production, and phonetic spelling, and learners' vocabulary sizes with the Vocabulary Levels Test (VLT). Learners attended 4th of Primary, were 9-10, had Spanish as their L1. CLIL learners had received 700 hours of English and traditional learners 419. Despite the difference in amount and nature of the input received, very similar results were obtained. The young age of the learners may impose certain cognitive constraints on expression and metalinguistic awareness that might override hours of instruction and the beneficial communicative nature of the CLIL approach.