Background: Approximately one-third of all borderline ovarian tumours (BOT) are diagnosed in patients with childbearing potential. Detailed information regarding their specific characteristics and prognostic factors is limited. Methods: Clinical parameters of BOT patients treated between 1998 and 2008 in 24 German centres were retrospectively investigated. Central pathology review and prospective follow-up were carried out. Patients <40 versus ≥40 years were analysed separately and then compared regarding clinico-pathological variables and prognosis. Results: A total of 950 BOT patients with a median age of 49.1 (14.1-91.5) years were analysed [280 patients <40 years (29.5%), 670 patients ≥40 years (70.5%)]. Fertility-preserving surgery was carried out in 53.2% (149 of 280) of patients <40 years with preservation of the primarily affected ovary in 32 of these 149 cases (21.5%). Recurrence was significantly more frequent in patients <40 years (19.0% versus 10.1% 5-year recurrence rate, P < 0.001), usually in ovarian tissue, whereas disease-specific overall survival did not differ between the subgroups. In case of recurrent disease, malignant transformation was less frequent in younger than in older patients (12.0% versus 66.7%, P < 0.001), mostly presenting as invasive peritoneal carcinomatosis. Multivariate analysis for patients <40 years identified advanced International Federation of Gynecology and Obstetrics (FIGO) stage and fertility-sparing approach as independent prognostic factors negatively affecting progression-free survival (PFS) while, for patients ≥40 years, higher FIGO stage and incomplete staging was associated with impaired PFS. Conclusions: Despite favourable survival, young BOT patients with child-bearing potential are at higher risk for disease recurrence. However, relapses usually remain BOT in the preserved ovaries as opposed to older patients being at higher risk for malignant transformation in peritoneal or distant localisation. Therefore, fertility-sparing approach can be justified for younger patients after thorough consultation.