OBJECTIVES Interval debulking surgery (IDS) following neo-adjuvant chemotherapy (NAC) is a treatment option in advanced ovarian cancer. It is recommended to perform IDS early, after 3 cycles of NAC, but late IDS (after 6 cycles) may yield better results. Delaying IDS, however, harbours the risk of loosing the opportunity for debulking surgery. STUDY DESIGN Retrospective comparison of two groups of patients with advanced ovarian carcinoma (stages IIC-IV) treated by platinum-based chemotherapy (CT) having undergone early IDS (after 3.6 cycles, group 1, n=33) or late IDS (after 6.3 cycles, group 2, n=104). Contemporary patients who had undergone standard treatment by primary debulking surgery (PDS)+CT (group 3, n=446) and those treated by CT alone (group 4, n=64 patients) served as internal controls. RESULTS Prognosis in IDS patients (groups 1+2) was comparable to that in PDS patients (group 3). Only a few patients in group 4 potentially had lost an opportunity for debulking surgery. Groups 1 and 2 were well-matched concerning usual prognostic factors. Surgery extent and post-operative outcomes were similar in both. In contrast, complete cytoreductions were significantly more frequent in late than in early IDS (group 2 vs.1: 58% vs. 36%, p=0.03) and survival was not inferior in the late IDS group compared to the early IDS group with 37 vs. 22 months, respectively (p=0.09). CONCLUSION Late IDS yields higher complete resection rates than early IDS and should be evaluated prospectively for outcome in further trials.