Patients with complications of chemotherapy, either acute or chronic, are frequently encountered in the emergency department (ED). Some patients present with complaints immediately after chemotherapy administration, whereas others may show subtle, secondary signs or may have no signs or symptoms of chemotoxicity. An increased index of suspicion prompts early recognition, diagnosis, and prevention of further iatrogenic injury. This article reviews characteristic hypersensitivity reactions, typical organ system dysfunction, and treatment strategies for adult patients who present to the ED with complications after chemotherapy.