There is controversy about patient outcomes and pathological parameters of prognostic significance in patients with stage II or stage III ovarian serous borderline tumors. Forty-nine cases of stage II and III ovarian serous borderline tumors were identified on review of the medical records at Vancouver Hospital and British Columbia Cancer Agency for the period from 1979 to 1996. Pathological features assessed included presence of micropapillary architecture, tumor cell DNA content (ploidy), and characteristics of the extraovarian implants, including invasiveness and mitotic activity. Clinical follow-up information (3-17 years of follow-up) was obtained for 48 patients. Fifteen patients had stage II tumors and 34 had stage III tumors. Fourteen patients experienced tumor recurrence 1 to 8 (mean 3.5) years after presentation and of these, six patients died of disease (2, 3, 4, 7, 10, and 11 years after presentation). Patients with gross residual disease, as assessed by the surgeon, more frequently experienced a recurrence compared with patients without gross residual disease, but this difference did not reach statistical significance (0.05<p<0.1). The patients who died of disease all had stage III tumors. Patients with invasive implants had a significantly worse outcome than patients with noninvasive implants (p<0.005). Other pathological features (ploidy, micropapillary architecture, invasiveness of implants, mitotic activity of implants) were not significantly predictive of tumor recurrence or death. No single pathological feature or combination of features was present in all patients who subsequently died of disease. In conclusion, the prognosis for patients with advanced-stage serous borderline tumors in this population-based study is very favorable, with only six patients ultimately dying of progressive disease. The combination of stage (stage III) and invasiveness of extraovarian implants identifies a small subset of patients with advanced-stage serous borderline tumors with a significantly worse prognosis, who may benefit from adjuvant therapy.