As advances in cancer therapy improve the prognosis of patients with childhood malignancies, awareness of the consequences of treatment methods assumes increasing importance. All cancer treatment modalities are associated with toxic effects, and the spectrum of therapy-induced complications involves all organ systems. Radiologists have a pivotal role in detecting these sequelae, which can be categorized by the affected organ system and by whether they occur (a) at diagnosis or during initial therapy or (b) after the completion of treatment. The first group consists of oncologic emergencies, infectious complications, and irritant effects. Oncologic emergencies can be further categorized as space-occupying lesions (e.g., superior vena cava syndrome or spinal cord compression), vascular abnormalities (e.g., hyperleukocytosis, anemia, coagulopathy), and metabolic emergencies (e.g., tumor lysis syndrome). Common complications developing after completion of treatment include leukoencephalopathy and neurocognitive defects; cataract formation; cardiomyopathy and congestive heart failure; hepatic dysfunction, fibrosis, and cirrhosis; radiation enteritis; renal dysfunction or failure; scoliosis and short stature; hypothyroidism; gonadal dysfunction; graft-versus-host disease; and development of secondary malignancies. Physician awareness of these complications will permit more effective patient surveillance, which may afford patients the opportunity for earlier intervention in these situations and improved quality of life.