Ovarian tumors of low malignant potential, often termed "borderline tumors," have been defined as those that have some but not all of the morphologic features of malignancy (i.e., they are not invasive). With the use of data obtained by the western Washington population-based Cancer Surveillance System for 1975-83, the incidence of serous and mucinous borderline epithelial ovarian tumors was analyzed, as well as the survival of women who developed them. The incidence of borderline tumors increased with increasing age, although at a pace somewhat slower than that of malignant ovarian tumors. There was an upward trend in the incidence of borderline tumors starting in the late 1970's, a trend not present for malignant tumors. Only 12% of borderline tumors were not confined to the ovary, as opposed to 40% of malignant Grade I and 73% of other malignant ovarian neoplasms. At 5 years following diagnosis, the survival of women with borderline tumors was 93% that of the general female population. This percentage varied little by stage or histologic type. Given the reduced survival of women with these ovarian tumors and the lack of a sharp histologic distinction between borderline and Grade I malignant lesions, it is recommended that borderline ovarian tumors be routinely ascertained by population-based cancer registries.