The antimetabolite mercaptopurine is commonly used as part of treatment regimens for acute lymphoblastic leukemia and as treatment for inflammatory bowel diseases. Adverse effects associated with mercaptopurine include myelosuppression, hepatotoxicity, and hyperpigmentation. We describe a 36-year-old man with Philadelphia chromosome-negative pre-B-cell acute lymphoblastic leukemia who experienced a serious mercaptopurine-induced hypersensitivity reaction requiring prolonged hospitalization and extensive laboratory testing and imaging. He was treated with a multiagent chemotherapy regimen. Mercaptopurine 60 mg/m(2)/day orally was started as part of his third course of chemotherapy. On day 9 of mercaptopurine therapy, the patient developed persistent fevers, skaking chills, and rigors that persisted in the absence of documented infection, consistent with drug fever. All symptoms and signs resolved after discontinuation of mercaptopurine. Use of the Naranjo adverse drug reaction probability scale indicated a probable relationship between the patient's development of fever and mercaptopurine therapy. Mercaptopurine-induced fever has been reported in patients with inflammatory bowel diseases, but this case report is the first, to our knowledge, in a patient with acute lymphoblastic leukemia. Health care professionals should be aware of the possible development of fever as a hypersensitivity reaction in patients with acute lymphoblastic leukemia treated with mercaptopurine.