Ovarian cancer patients in whom complete tumor removal is impossible with primary debulking surgery (PDS) may benefit from neoadjuvant chemotherapy and interval debulking surgery. However, the task of performing a pre-operative evaluation of the feasibility of PDS is difficult. We aimed to investigate whether the risk of malignancy index (RMI) was a useful marker for this evaluation. RMI and surgical outcome were investigated in 164 patients, 49 of whom had no residual tumor after PDS. The receiver operating characteristic curve showed an area under the curve of 0.72 (confidence interval: 0.64–0.80). The possibility of complete tumor removal decreased with increasing RMI and there was a tendency towards higher RMI in patients with residual tumor after PDS, but no single cut-off value of RMI produced useful clinical predictive values. In conclusion, RMI alone is not an optimal method to determine whether complete tumor removal is possible with PDS.