We report the case of a 14 year old girl who presented with a non-metastatic Ewing's sarcoma involving her superior pubic ramus. She received 14 courses of alkylating agent-based chemotherapy and direct radiation to her hemi-pelvis (55 Gy) and is alive and disease-free 8 years later. Multiple biopsies of ovarian cortical tissue were cryopreserved, with her written consent, before treatment began. Ovarian failure was confirmed on completion of treatment with cessation of menses and persistently elevated serum gonadotrophin and low estradiol levels on repeated measurement over 2 years. HRT was initiated. Irregular vaginal bleeding occurred due to radiation vaginitis. Reimplantation of ovarian cortical tissue was considered at 19 years as fertility was desired, but the decision deferred. A spontaneous conception occurred 1 year later and a healthy boy (birthweight 2.9 kg, 3rd-10th centile) was delivered at term by elective Caesarean section. This is the first case of a spontaneous conception occurring in a young woman with documented ovarian failure in whom ovarian cortical tissue had been cryopreserved. Clinicians should be aware of the possibility of spontaneous conception despite confirmed ovarian failure in young women successfully treated for cancer.