Side effects have to be monitored carefully in all patients receiving drugs profoundly affecting their metabolism. Corticosteroids produce the well-known Cushing's syndrome. Children rarely develop gastric ulcers or disturbances of haemostasis. If steroids are administered for more than one year, the eyes have to be examined for signs of cataract. Rarely, the pseudotumor cerebri syndrome (increased intracranial pressure) may be observed. A monitoring schedule adapted from adult medicine for paediatric purposes is proposed in the text. Nowadays, mostly combinations of cytotoxic drugs are used. Often, doses are as high as the patient can tolerate them. Effects of these drugs are especially marked upon rapidly proliferating tissues such as the haematopoietic system and gastrointestinal mucosa. Immunosuppression may have very serious consequences. Infections caused by organisms usually considered as little pathogenic (e.g., pneumocystis carinii) may become deleterious; some infections may remain concealed or produce fever as the only symptom. Particular side effects of certain cytotoxic drugs are described in the text. Many therapeutic regimens are now standardized, as are monitoring schedules. Frequency and nature of the necessary controls depend upon the disease treated and the kind of therapy applied, as well as on its intensity. Both hospital and family doctors are indispensable for this surveillance; they must remain in close contact with each other.