In this longitudinal study, we examined the effects of the use of English as the language of instruction in the first 4 years of secondary education in The Netherlands on the students' language proficiency in English and Dutch, and achievement in subject matters taught through English. Compared to a control group in regular secondary education, students attending this form of bilingual education showed higher scores for their English language proficiency in terms of oral proficiency and reading comprehension. No effects have been found for receptive word knowledge and no negative effects have been found with respect to the results of their school leaving exams at the end of secondary education for Dutch and subject matters taught through English. (Contains 5 tables and 1 figures.)