Purpose: Differences of the clinical features of Stage I borderline ovarian tumors and Stage I ovarian cancer need to be clarified. Methods: We retrospectively investigated 215 patients with Stage I ovarian tumors (67 with borderline tumors and 148 with ovarian cancer) treated between 1988 and 2001. Results: Only one patient with a borderline tumor developed recurrence, while recurrence was found in 20 patients with Stage I ovarian cancer. There was a significant difference in the recurrence rate between patients with Stage Ia or Ib ovarian cancer and those with Stage Ic cancer (p = 0.007). Clear cell adenocarcinoma showed a higher recurrence rate. Among our patients with recurrence, only five in whom the recurrent tumor could be surgically resected are currently alive and disease-free. Conclusions: This study confirmed the low aggressiveness of Stage I borderline ovarian tumors and high aggressiveness of Stage Ic ovarian cancer or clear cell adenocarcinoma. In patients with recurrence, surgical resection may improve survival.