Tumor lysis syndrome in a patient with metastatic Merkel cell carcinoma. A first course of combination chemotherapy for large volume metastatic disease in a patient with Merkel cell carcinoma resulted in a tumor lysis syndrome. After this course a nearly complete response was documented. This case report extends further the chemosensitivity of Merkel cell carcinoma and demonstrates the need for tumor lysis syndrome prophylaxis in patients with bulky disease or fast-growing tumors. 