Background: Treatment of metastatic breast cancer experienced significant improvement in the past decades by introduction of highly effective therapies, but survival still remains poor. Nonetheless, in some patients, long-term survival can be achieved by sequent endocrine and chemotherapy treatment, although toxicity and resistance eventually occur, until no further suitable and approved therapies remain. If further therapy is needed, therapist may be forced to consider treatments which are promising but not approved, such as the alkylating agent treosulfan, which is approved for the treatment of ovarian cancer only. Thus, relevant clinical data on its use in human breast cancer are lacking. Case Report: The authors report the case of a 49-year-old woman with heavily pre-treated, metastatic breast cancer, who experienced complete remission of pulmonary and soft tissue metastases while under treatment with treosulfan. Treatment was generally well-tolerated. Conclusion: Treosulfan might be an effective and well-tolerated treatment even in heavily pre-treated patients with metastatic breast cancer.