Content and Language Integrated Learning (CLIL) is becoming a very popular approach in Europe and many other parts of the world. However, due to the novelty of its implementation the number of longitudinal studies is scant. With this in mind, the overall aim of this study was to examine the effect of CLIL sessions and their intensity on the learning of English as a foreign language. Two test rounds were conducted in a longitudinal study spanning one year. The participants were 393 secondary education students enrolled in both bilingual and monolingual regions in Spain. Our findings revealed a significant impact of the number of CLIL sessions on students’ proficiency in English.