This paper analyses the written competence attained by two groups of bilingual students that follow two different CLIL programmes, and another group enrolled in a traditional English as a Foreign language (EFL) programme. This study also analyses the longitudinal progression of these three groups to offer a more prolonged perspective on CLIL. Our results show the CLIL groups score better in relation to the five categories analysed in written production: content, organisation, vocabulary, language usage and mechanics, which suggests there is a positive relationship between the amount of exposure through English and written foreign language proficiency. Furthermore, the longitudinal evaluation of the results show that students enrolled on CLIL programmes outperform students on the EFL programmes, and this advantage increases with grade, confirming the effectiveness of the CLIL approach on written production outcomes. These results serve as evidence that CLIL can be more useful than traditional language teaching in promoting proficiency in the foreign language.