There is currently no standardized therapy available for metastatic breast cancer in patients with aromatase inhibitor (AI)-resistant breast cancer. We conducted a prospective study to examine the efficacy and safety of high-dose toremifene (TOR) treatment for the first-line treatment of metastatic breast cancer following AI adjuvant therapy. A multicenter phase II study was designed (Registry no.: UMIN000000489). Inclusion criteria comprised hormone-responsive postmenopausal women who had received adjuvant AI postoperatively for >1 year and had relapsed during the treatment or within 12 months of completion of adjuvant therapy. Treatment comprised oral intake of 120 mg TOR once a day. The primary endpoint was objective response rate (ORR). The secondary endpoints were evaluations of clinical benefit (CB), progression-free survival (PFS) and toxicity. A total of 13 patients were enrolled. ORR was 7.7% (1/13) [95% CI, 0.2-36.0%]. In total, 7 patients (53.8%) had stable disease (SD), 5 of whom were long SD, and 5 patients (38.5%) experienced progressive disease (PD). The CB rate was 46.2% (6/13) [95% CI, 19.2-74.9%]. The median time to PFS was 5.9 months. No serious adverse events were observed. Patients with HER2-positive disease exhibited marginally poorer PFS (p=0.08). Patients with PD had a relatively short duration of AI treatment in contrast to responders, who had a longer period of AI treatment (p=0.02). High-dose TOR as a first-line treatment following AI adjuvant therapy was effective and well tolerated. A longer duration of adjuvant AI therapy and negative HER2 overexpression may, with further studies, be beneficial as positive predictive factors for the effectiveness of TOR treatment.