Reports describing the behavior of micropapillary serous carcinomas (MPSCs) of the ovary have focused on those that are noninvasive. There are only very limited data on the behavior of those that are invasive. To further characterize the behavior of MPSCs, invasive versus noninvasive primary tumors were distinguished based on the presence or absence of destructive infiltrative growth. To qualify for inclusion, invasive MPSCs, like the noninvasive tumors, were required to display a micropapillary architecture and low-grade nuclei. A total of 135 cases of MPSC were identified: 96 noninvasive and 39 invasive. On follow-up, survival for 10 patients with stage I noninvasive and invasive MPSCs was 100%, and survival for women with stage II and III noninvasive and invasive MPSCs with noninvasive implants was 80%. In contrast, the 5-year and 10-year survival for patients with stage II and III noninvasive MPSCs with invasive implants was 85% and 55%, respectively. The 5-year and 10-year survival for women with invasive MPSCs and invasive implants was 55% and 45%, respectively. The median time from diagnosis to death for women with noninvasive and invasive MPSCs with invasive implants was 60 months (range 33-240 months). The indolent behavior of these low-grade carcinomas distinguishes them from conventional serous carcinomas, which are high-grade aggressive neoplasms. Five of six patients with small (<5 mm) MPSCs in whom follow-up was available presented with high stage disease. Of these five women, three are alive and well and two are alive with disease (one with invasive and one with noninvasive implants). Nearly three fourths of noninvasive MPSCs were associated with atypical proliferative serous tumors, adenofibromas, or both, and 62% of invasive MPSCs were associated with noninvasive MPSCs, atypical proliferative serous tumors, and adenofibromas, alone or in combination. In addition to the frequent mixtures of these tumor components, transitions between them were common. These data in conjunction with recent molecular genetic studies strongly suggest that MPSCs (low-grade carcinomas) arise from atypical proliferative serous tumors unlike conventional serous carcinomas (high-grade carcinomas), which appear to develop de novo. The findings provide further support for the hypothesis that there are distinct pathways of carcinogenesis for low-grade and high-grade serous carcinoma of the ovary.