Borderline ovarian tumors (BOTs) make up around 10–20% of all epithelial ovarian tumors. The aim of the present study was to investigate the outcome of a complete large population-based cohort of patients treated for BOT. All patients (n= 399) treated for BOT in the western part of Sweden (population around 1.6 million) between 1993 and 2004 were followed. The treatment consisted of primary staging surgery with addition of platinum-based adjuvant chemotherapy for the majority of aneuploid tumors. Data relating to the surgical procedure, FIGO stage, histopathology, ploidy status, adjuvant chemotherapy, and disease state (recurrence or death) at follow-up visits were continuously entered into a cancer quality registry. Data concerning cases and deaths were also controlled against the Swedish National Cancer Registry. The median age of the BOT patients was 55 years (range 16–90). The relative 5- and 10-year survivals were 99.9% (95% CI 96.3–102.4) and 103.5% (95% CI 97.2–108.2), respectively. Aneuploidy was found in 63 (17%) patients, with significantly more aneuploid tumors found among patients of older (>60 years) age. Out of the 399 patients, 8 had recurrence of the disease. Three of the eight patients died from the disease. Five patients with recurrence are alive, three of these patients with no signs of disease after additional treatment. This complete long-term follow-up of a large population-based cohort of BOT patients shows that there is a good overall survival in this patient group.