Background: Over the past 10 years the number of newly diagnosed HIV infections per year in Poland has been increasing. International data shows that in-school education about prevention of HIV leads to improved knowledge, increased condom use and safer sex attitudes. Data about HIV knowledge in polish schools is lacking. Objective: The aim of the study was to assess the knowledge of polish secondary school students regarding HIV/AIDS, and how it changes after one training in this topic. Material and methods: An online questionnaire with 30 questions about HIV/AIDS and sources of knowledge was given to students in two secondary schools in Warsaw, Poland. After completing the questionnaire an educational training was conducted. The same online questionnaire was distributed to the same classes after a year from the first survey and training. Numbers of correct answers were counted and the change of correct answers between first and second questionnaire was calculated. Results: In the first part of the study n=364 students filled in the questionnaire and in the second part n=257. The majority of students self-assessed their knowledge as not sufficient (84.3%). Questions answered incorrectly by most of the respondents concerned routes of transmission, pre-exposure prophylaxis (PrEP) and post-exposure prophylaxis (PEP). In 16/20 yes/no questions an improvement in knowledge was noted in the second survey. Conclusions: This study showed some knowledge gaps about HIV/AIDS of surveyed students. More education about prevention of HIV/AIDS should be included in the school programme. A one-time educational training can be beneficial.