This paper presents results from a study with the following research questions: Are pupils' opinions on genetically modified organisms (GMOs) influenced by biology teaching? What is important for the opinion pupils hold and how does knowledge work together with other parameters such as values? 64 pupils in three upper secondary schools answered questionnaires, in which they expressed opinions and gave arguments on applications of GMOs, before and after biology courses. The pupils’ knowledge of genetics and GMOs was also investigated. Eleven pupils were then interviewed in order to go into argumentation in more depth. Results from questionnaires show ore pupils were positive about genetically modified tomatoes after the courses. No correlation was found between knowledge of basic genetics and opinion. Most of the pupils could express arguments for and against the applications but they built their personal opinion on different arguments. An important concern was potential risks. This was confirmed in the interview. Few had any idea of how the different applications are risk assessed or how scientists work. Depending on risk judgement and/or how they trusted scientists, the pupils came to different opinions. Other important factors for opinions were the purpose of the application, the time perspective and emotions.