Bilingual education programs, which consist of doing a substantial part of the instruction in a language different from the native language of the students, exist in several countries like the United States, India, and Spain. While the economic benefits of knowing a second language are well established, the potential effects over the learning of other subjects have received much less attention. We evaluate a program that introduced bilingual education (in English and Spanish) in primary education in a group of public schools of the Madrid region in 2004. Under this program, students not only study English as a foreign language but also some other subjects (at least Science, History, and Geography) are taught in English. In order to evaluate the program, a standardized test for all sixth grade students in Madrid on the skills considered “indispensable” at that age is our measure of the outcome of primary education. Our results indicate that there is a clearly negative effect on the exam results for the subject taught in English, for children whose parents have less than upper secondary education. This negative effect is a composite of two phenomena: the effect of the program on the student's knowledge of the subject and a reflection of the student ability to do the test in their native language when English is the medium of instruction. Although we are not able to separate quantitatively these two effects, the composite effect has a relevant interest, because the results for exams taken in Spanish are the measures that determine academic progression in the Spanish system. In contrast with the previous result, there is no significant effect for anyone on mathematical and reading skills, which were taught in Spanish. (JEL H40, I21, I28)