Postoperative nausea and vomiting (PONV) occurs in every third patient undergoing general anesthesia without PONV prophylaxis. Antiemetic prophylaxis with dexamethasone is commonly used in patients at moderate risk. We present a case in which PONV prophylaxis with a single dose of dexamethasone led to tumor lysis in a patient with acute leukemia. In case of a cancer patient at moderate risk for PONV, the anesthesiologist should contact the oncologist first, or use other antiemetic drugs such as antiserotoninergic agents for PONV prophylaxis.