Objective: To study the impact of first-line antineoplastic treatment on ovarian reserve in young girls returning for ovarian tissue cryopreservation (OTC) in connection with a relapse. Design: Retrospective case-control study. Setting: University hospitals. Patient(s): Sixty-three girls under the age of 18 years who underwent OTC before (group 1: 31 patients) and after (group 2: 32 patients) their initial cancer treatment. Intervention(s): None. Main Outcome Measure(s): Follicular densities (follicles/mm 3 ) measured from an ovarian cortical biopsy before OTC. The ovarian volume (mL) of entire ovaries excised for OTC was also monitored. Result(s): There was no statistically significant difference in the mean age or follicular density between groups 1 and 2 (334 AE 476/ mm 3 vs. 327 AE 756/mm 3 ). In contrast, the ovarian volume and total number of ovarian cortex chips cryopreserved were statistically significantly lower in patients who received gonadotoxic treatment before OTC (mean AE standard deviation [SD]: ovarian volume, 5.3 AE 3.1 mL vs. 2.9 AE 2.1 mL, respectively; number of cortex chips: 21.3 AE 8.1 vs. 15.2 AE 7.1, respectively). The reduction in the estimated ovarian reserve ranged from 10% to 20% in children to around 30% in adolescent girls (>10 years). Conclusion(s): Girls under the age of 10 tolerate a gonadotoxic insult better than adolescents, who may experience up to a 30% reduction in the ovarian reserve via first-line gonadotoxic treatment, which at present is considered to have little effect on the follicle pool. This information will improve counseling of young female cancer patients in deciding whether to undergo fertility preservation treatment. (Fertil Steril Ò 2016;106:1757-62.