With the aim of clarifying the nature of the synergistic action of LNT with endocrine therapy for mammary tumor, the effect of LNT post-treatment was investigated in rats with DMBA-induced mammary tumors and also in patients with recurrent breast cancer. LNT injection following surgical therapy (Ax + Ox) resulted in a much greater regression of tumor growth than that obtained by surgery alone, but not after medical therapy (Tamoxifen). Image analysis using an immunohistochemical technique revealed that LNT-surgical therapy resulted in marked atrophy of tumors and as well as intense infiltration of T cells, B cells and macrophages into the stroma around the tumor. Blood prolactin level was greatly reduced by LNT injection. A clinical randomized controlled study demonstrated the efficacy and safety of LNT post-treatment with surgical therapy in 33 patients with recurrent breast cancer. The mode of the synergistic action of LNT in combination with endocrine therapy on hormone-dependent tumors was discussed.