Ovarian tumors of low malignant potential (LMPs) are intermediate between adenomas and ovarian carcinomas (OCs); however, whether an LMP is a precursor of OC or is a unique type of tumor that will not progress to OC is still controversial. To gain better insight into the relationship between LMP and OC, we compared proliferative and apoptotic activity and tumor size in 50 cases of LMP and 27 cases of OC. We employed immunohistochemical staining with Ki-67 and apoptotic labeling was based on the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling. The average tumor size observed in mucinous LMP was significantly larger than that in mucinous OC (p = 0.014). In contrast, there was no difference in tumor size between serous OC and serous LMP. The average Ki-67 labeling index (LI) in mucinous OC was significantly higher than that in mucinous LMP (p = 0.028). There was no difference between serous OC and serous LMP in the Ki-67 LI. There was also no difference in apoptotic indeed (AI) between OC and LMP, irrespective of histological subtype. A significant positive relationship between the Ki-67 LI and tumor size was observed in mucinous LMP (R = 0.516, p = 0.0003). Interestingly, the Ki-67 LI was inversely correlated with tumor size in mucinous OC (R = 0.570, p = 0.0317). Significant positive correlations between Ki-67 LI and tumor size were observed in both serous LMP and serous OC (R = 0.707, p = 0.0488, R = 0.789, p = 0.0168, respectively). No association was seen between AI and tumor size in LMP and OC, irrespective of histological subtype. The different associations between proliferative activity and tumor size in the mucinous subtype of LMP and OC suggest that mucinous LMPs may not be precursors of mucinous OCs. In contrast, the similar associations of proliferative activity with tumor size in the serous subtype of LMP and OC suggest that serous LMPs may be precursors of serous OCs.