Tumor lysis syndrome (TLS), resulting from massive necrosis of neoplastic cells after chemotherapy, is a rare complication in nonhematologic malignancies. A 32-year-old woman suffering from a rapidly progressing breast adenocarcinoma metastatic to the liver and bones received a course of single-agent chemotherapy with mitoxanthrone and 4 days later developed the tumor lysis syndrome, and subsequently acute renal failure. The patient responded well to appropriate treatment. This case report points out that breast cancer can be extremely sensitive to chemotherapy and suggests that prophylaxis for tumor lysis syndrome should be considered in the subset of patients with breast carcinoma who have hepatic metastases and large tumor burdens.