Purpose/aim: Modified radical mastectomy is the standard surgery for breast cancer in developing countries. However, seroma formation regarded as the most frequent postoperative complication limits the therapeutic benefit of mastectomy and axillary surgery. The purpose of this study was to evaluate the efficacy of OK-432 in reducing seroma formation after axillary dissection. Methods: This prospective cohort study included 80 patients with advanced breast cancer who underwent modified radical mastectomy. Patients were randomized into two groups, which differed with the OK-432 administration. N = 40 patients per group were treated with either OK-432 plus closed suction drainage or drainage-only. Result: In comparison with the drainage-only group, we found that patients in the OK-432 group had a lower drainage volume (p = .030) and a shorter duration of axillary drainage (p < .01). Besides, the use of OK-432 could reduce the incidence of seroma formation (p < .01) and the volume of seroma (p = .040). There were also significant differences in reducing the chance of evacuative punctures (p = .036) and the healing time (p < .01) between control and OK-432 group. Conclusion: OK-432 not only shortened the suction drainage duration, but also significantly reduced seroma formation as well as the needs for aspiration punctures after modified radical mastectomy.