This paper reports on the effectiveness of bilingual education in the Netherlands. After a brief history of the rise of bilingual education in the Netherlands, the study traces the development of English proficiency of two cohorts at Dutch high schools during one year: a group of Year 1 students (average age 12) and a group of Year 3 students (average age 14) were tested three times during one academic year. The results suggest a dynamic interplay as proficiency increases between condition and other factors such as initial proficiency, scholastic aptitude, out of school contact, and motivation/attitude factors. In Year 1, scholastic aptitude and initial proficiency were strong predictors for all students. In Year 3, scholastic aptitude no longer played a role, but initial proficiency and motivation/attitude did. The students who received bilingual education outperformed the students from the other two groups (regulars and controls).