This study compares the language proficiency gains of two groups of students taking a business English course module in a bilingual university in Catalonia (Spain). Whereas one of these groups followed a 'translanguaging' or 'plurilingual' pedagogy, the other followed a strictly monolingual approach. Participants were 54 mostly Catalan/Spanish bilingual university students of Business (n = 35 translanguaging and n = 19 monolingual). Whereas the teacher in the 'translanguaging group' used and fostered the use of Catalan and Spanish besides English, the teacher in the 'monolingual group' only used English and allowed only English in class. Participants were administered a placement test, and performed a written composition and an oral sales pitch the first and last week of the semester. Participants were also administered a questionnaire before and after the treatment. The EFL development of the participants was measured in terms of fluency, lexical complexity, grammatical complexity and accuracy, but it was also assessed by an expert examiner, who based her ratings on a rubric including four scales: language, communicative achievement, content, and organisation. Results show that both groups experienced comparable gains, but the few significant differences favoured the translanguaging group.