An 83-year-old woman with no previous history of malignancy was admitted to our institution with weakness and anemia and subsequently developed acute tumor lysis syndrome secondary to newly diagnosed Burkitt's leukemia/lymphoma. This syndrome has been previously described in patients with hematologic malignancies; however, its development has been related to the administration of chemotherapy, steroids, or radiotherapy. The spontaneous occurrence of tumor lysis syndrome has not been previously reported; however, Cohen et al. [Am J Med 58:486-491, 1980] report 8 of 37 patients with "clinically insignificant pretreatment derangements" of serum potassium, phosphate, and calcium.