Background: Allogenic hematopoietic stem cell transplantation (allo-HSCT) is associated with increased risk of late effects, including infertility. The use of total body irradiation in conditioning is common in many countries. However, in Norway myeloablative chemotherapy is more commonly used, which makes Norwegian survivors interesting. The purpose of this project thesis is to examine fertility after stem cell transplantation in children and young adults. Material and methods: The data presented is collected from the AlloSS-young study, a cross sectional study of Norwegian long-term survivors treated with allo-HSCT. A total of 102 participants from AlloSS-young, 55 females and 47 males, who participated at the questionnaire and the clinical examination were included. The participants fertility was examined with a descriptive focus on menstrual patterns, semen analysis and achieved pregnancies, both natural and medically assisted. Results: Among female participants ceased menstruation was reported to a mean of 26,6 years, while 22/51 (mean 28,4 years at AlloSS-young, range 18-50) still had menstruation when answering the questionnaire. Decreased numbers of sperm were proven in 22/38 samples, of which 14 contained no sperm. Among the participants, 17 achieved natural pregnancies, while 17 did not achieve pregnancy within a year of attempts. Medical assistance was attempted by 10 participants, and five became pregnant. In total 28 children were born. Conclusion: The results presented in the project thesis regarding allo-HSCT in young patients and affected fertility are in accordance with results from similar studies. This substantiates the importance of focus on fertility and fertility preservation measures before, during and after treatment.