Purpose The purpose of this study was to examine outcomes associated with hormonal maintenance therapy (HMT) compared with routine observation (OBS) after primary cytoreductive surgery and platinum-based chemotherapy in women with stage II to IV low-grade serous carcinoma of the ovary or peritoneum. Patients and Methods Eligibility criteria for patients from our database were: treatment with primary surgery followed by platinum-based chemotherapy, stage II to IV disease, at least 2 years of follow-up for patients who had not experienced recurrence, and adequate clinical information. The two groups were compared for progression-free survival (PFS) and overall survival, and a multivariable Cox regression analysis was performed. Subset analyses for patients who were disease free or had persistent disease were also performed. Results Between 1981 and 2013, 203 eligible patients-133 who underwent OBS and 70 who received HMT-were seen at our institution. Median PFS for patients who underwent OBS was 26.4 months, compared with 64.9 months for those who received HMT ( P < .001). No statistically significant difference in overall survival was observed between the two groups (102.7 v 115.7 months, respectively). For subgroups of women who were disease free or had persistent disease, median PFS was superior for those who received HMT (81.1 v 30.0 months; P < .001 and 38.1 v 15.2 months; P < .001, respectively). Women who received HMT had a significantly lower risk of disease progression compared with those who underwent OBS (hazard ratio, 0.44; 95% CI, 0.31 to 0.64; P < .001). Conclusion Women with stage II to IV low-grade serous carcinoma who received HMT after primary treatment had significantly longer PFS compared with women who underwent OBS. These findings warrant further investigation using a prospective trial design.