The first clinical case of a patient with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) was presented by Schwartz et al. in 1957 (Am J Med 1957; 23: 529-42), describing two patients with lung cancer who developed hyponatraemia associated with continued urinary sodium loss. They postulated that the tumours led to the inappropriate release of antidiuretic hormone (ADH), later discovered to consist of arginine-vasopressin (AVP). This suggestion was later confirmed in several studies. The clinical description of the syndrome has changed little since the original observation, and the cardinal findings of SIADH are as follows: (i) hyponatraemia with corresponding hypo-osmolality of the serum and extracellular fluid, (ii) continued renal excretion of sodium. (iii) absence of clinical evidence of fluid volume depletion, (iv) osmolality of the urine greater than that appropriate for the concomitant osmolality of the plasma, i.e. urine less than maximal diluted, and (v) normal function of kidneys, suprarenal glands and thyroid glands. Measurement of AVP in plasma is not a part of the definition of SIADH. SIADH may be caused by a variety of malignant tumours, but may also be caused by various other conditions, such as disorders involving the central nervous system, intrathoratic disorders such as infections, positive pressure ventilation and conditions with decrease in left atrial pressure. Also, a large number of pharmaceutical agents have been shown to produce SIADH, including a number of cytotoxic drugs such as vincristine, vinblastine, cisplatin, cyclophosphamide, and melphalan. A broad spectrum of malignant tumours has been reported to cause SIADH; however, most of these observations have been in case reports including very few patients. This includes a number of primary brain tumours, haematologic malignancies, intrathoracic non-pulmonary cancers, skin tumours, gastrointestinal cancers, gynaecological cancer, breast-and prostatic cancer, and sarcomas. Larger series of patients have revealed that SIADH occurs in 3% of patients with head and neck cancer (47 cases out of 1696 patients), in 0.7% of patients with non-small-cell lung cancer (three cases out of 427 patients), and in 15% of cases of small-cell lung cancer (214 cases out of 1473 patients). The optimal therapy for SIADH is to treat the underlying malignant disease. If this is not possible, or if the disease has become refractory, other treatment methods are available such as water restriction, demeclocycline therapy, or, in severe cases, infusion of hypertonic saline together with furosemide during careful monitoring.