The effect of immunotherapy using sizofiran (SPG) on the prognosis of patients with ovarian cancers was prospectively studied in a total of 68 patients, who were randomly assigned to either a cisplatin, adriamycin and cyclophosphamide (PAC) therapy group or a PAC plus SPG combination therapy group. The survival rate was significantly higher in patients with stage Ic, II or III cancers treated with the PAC plus SPG combination, compared with the patients treated with PAC alone. In the SPG-receiving patients with stage Ic or more advanced cancers who were treated with four cycles or more of PAC, the outcome was improved (Cox-Mantel, p = 0.074; generalized Kruskal-Wallis, p = 0.032). Similar improvement was also observed in the patients with non-serous adenocarcinomas (Cox-Mantel, p-0.076; generalized Kruskal-Wallis, p = 0.045). No side effects attributable to SPG were recorded. The present results suggest that the use of SPG in combination with long-term chemotherapy improves the postoperative prognosis in ovarian cancer patients.