Low-grade serous carcinoma (LGSC) of the ovary has only recently been recognized as a disease entity distinct from the more common high-grade serous carcinoma (HGSC). When confined to the ovary, LGSC is associated with a very favorable prognosis, and chemotherapy is typically not recommended. There is little available information on the prognosis of patients with LGSC with extraovarian spread, from population-based tumor registries where there has been full pathology review. Thirty-two cases of Stage II to IV ovarian LGSC were identified in the Cheryl Brown Ovarian Cancer Outcomes Unit (1984–2000). In 19 cases, blocks were available and immunostaining for p53, p16, Ki-67, WT1, and E-cadherin was performed. Expression of these markers was then compared with a series of >400 cases of HGSC from the outcomes unit. LGSCs presented at Stage II in 10/32 cases, Stage III in 21/32 cases, and Stage IV in 1/32 cases. On follow-up, most patients died of disease, with <30% survival at 10 years. Compared with HGSCs, the LGSCs were significantly less likely to express p16 at high levels, or to show abnormal p53 expression (P=0.049 and <0.0001, respectively). Ki-67 staining indices were lower in the LGSCs (P<0.0001). There were no significant differences between LGSCs and HGSCs with respect to expression of WT1 and E-cadherin (P=0.27 and 0.62, respectively). This population-based series of LGSC with extraovarian spread at presentation had an unfavorable prognosis, similar to that of HGSC. As previously reported, LGSC shows lower tumor proliferation and fewer p53 abnormalities than in HGSC.