The aim of the study was to investigate how successfully pupils had learned content in content and language integrated learning (CLIL) and to assess pupils' affective learning factors, such as motivation and self-esteem, in CLIL. Learning was presented in terms of achievement level, which was described as the relationship between measured levels of intelligence and school success. The study indicated that there were no major differences in learning, whether the language used in instruction is the pupils' mother tongue or a foreign language; pupils of different intelligence levels had similar chances to succeed in both cases. However, it was discovered that there may not be as many overachievers among the pupils in CLIL as among the pupils in the Finnish language instruction group. Achievements in the Finnish language, i.e. the mother tongue, were not negatively affected by CLIL. In terms of the affective factors, the study indicated that CLIL pupils had a low self-concept in foreign languages, although pupils had a strong motivation to learn.