OBJECTIVE: Gonadal dysfunction is a major late complication after cancer diagnosis and treatment. We aimed to study the prevalence of premature ovarian insufficiency (POI) and the potential reduction of ovarian reserve in a cohort of adolescent and young adult (AYA) patients undergoing cancer treatments, evaluating ovarian function and reserve markers. We also aimed to analyze how these markers are related to each other and to treatment-related risk factors. METHODS: We performed a retrospective study, including all female AYA patients undergoing cancer treatment during childhood or adolescence, who visited our pediatric gynecology outpatient clinic between January 1, 2018, and August 30, 2023. Serum levels of anti-Mullerian hormone (AMH) and follicle-stimulating hormone (FSH), antral follicle count (AFC) and mean ovarian volume were evaluated. The correlations between these markers and how their alterations are related to treatment-related risk factors were analyzed. RESULTS: The prevalence of POI in 95 patients was 18.9%. A significant positive correlation was observed between AMH levels and both AFC and ovarian volume. AMH was the most reliable serum marker for ovarian function in terms of POI. Independent risk factors for ovarian dysfunction in relation to all the markers analyzed were hematopoietic stem cell transplantation (HSCT) and high doses of alkylating agents (≥6000 mg/m2). CONCLUSION: Gonadal dysfunction and infertility are quite common in AYA patients undergoing cancer treatment. High-dose alkylating agents and HSCT are the independent risk factors. AMH and FSH values, AFC and mean ovarian volume provide different but consistent information for closely monitoring patients after cancer treatment.