Abstract Study question What are the experiences and attitudes of Japanese teenagers toward sex and fertility education? Summary answer The participants received sex education on different items to different degrees, and 70% evaluated that the sex education they received in school as neutral. What is known already Although many studies have revealed younger people’s experiences of sex education and their knowledge regarding fertility, very few, both domestic and international, have identified teenagers’ experiences and attitudes toward fertility education. Since 2018, content on fertility has been included in supplementary reading books for Japanese upper secondary school students, and from 2023, students have begun to learn about fertility with the ‘Health’ textbook based on the revised national curriculum. Therefore, it is necessary to clarify teenagers’ experiences and attitudes toward learning about fertility in school. Study design, size, duration This was a cross-sectional questionnaire survey study. The target group consisted of students who had just enrolled to university, were 18-19 years old, and graduated from upper secondary high school in March 2023. The survey was conducted between July and August 2023. The study protocol was approved by the Ethics Committee (approval date: 2023. 6.1; approval number: 5-1). Participants/materials, setting, methods In liberal arts classes with approximately 80% or more first-year students, 873 students who agreed to participate in this study answered a Google forms questionnaire. Of these, 759 (86.9 %) met the inclusion criteria and were included in the analysis. We used the questionnaire ‘Teenager’s knowledge and attitudes to fertility’ which includes 25 items on whether students received and learned from sex education. Simple aggregation and chi-squared analyses were performed. Main results and the role of chance Regarding sex, 46.4% of the participants were male, 51.1% female, and 2.5% other. Of the participants, 99.2% had received sex education at school; 21.6% rated it as very good or good, 71.3% rated it as neutral, and 7% rated it as poor or very poor.The sex education content received by over 60% of the participants was related to the following: sexually transmitted infections (98.4%), contraception (92.5%), puberty (94.2%), menstrual cycle (90.7%), ovulation (87.6%), pregnancy (87.1%), and abortion (60.5%). Regarding fertility and related content, few participants received education on these in school: fertility (8.8%), fertility window (0.0%), menopause (18.3%), relationship between fertility and lifestyle (34.5%), endometriosis (8.8%), and PCOS (2.6%). Women learned more about the menstrual cycle in school compared to men (p<.05). This was the only significant gender difference.Outside school, 30% or less of the participants had studied all 25 items, and nearly 40% had never studied any of the items. Moreover, 11 items were studied by significantly few students outside school than in school (p<.01). Limitations, reasons for caution A limitation of this study is that only 18-19 year olds who had just entered a national university in 2023 were targeted. Therefore, caution should be exercised when generalising these results to the general Japanese teenage population. Wider implications of the findings Few teenagers have studied fertility and related topics both inside and outside of school. It is necessary to enhance schoolteachers’ abilities to appropriately educate teenagers about fertility. In addition, it would be effective for teenagers to be provided opportunities to learn about fertility outside school through websites or short movies. Trial registration number non applicable