Hyponatraemia is very common in AIDS patients. It is observed in about 40-50% of hospitalized patients. It may contribute to overall mortality in advanced disease. Vasopressin measurements in these patients basically present two distinct syndromes: hyponatraemia and 'normal' vasopressin levels (i.e. measurable vasopressin) and hyponatraemia with suppressed vasopressin. Hyponatraemia with suppressed vasopressin is very rare and has only been observed in AIDS patients with dementia and primary polydipsia. Hyponatraemia and measurable vasopressin can be also divided into two syndromes. In some patients vasopressin is 'appropriately' elevated, i.e. in those with body fluid losses (diarrhoea) or chronic hypovolaemia (adrenal failure); these patients also present with hyperuricaemia and other signs of low blood volume. In other patients vasopressin is 'inappropriately' elevated in those with no clinical evidence of hypovolaemia (typically characterized by low serum uric acid levels) such as in Pneumocystis carinii pneumonia and other opportunistic infections leading to SIADH. CSWS is a relatively frequent complication in some patients with cerebral infection or tumour. High-dose trimethoprim (for Pneumocystis carinii prevention) acts as an amiloride-like drug and induces a clinical state characterized by hyponatraemia and hyperkalaemia which is indistinguishable from hyporeninaemic hypoaldosteronism. The mechanism of the hyponatraemia caused by other drugs (miconazole, pentamidine, amphotericin, vidarabine) is not as yet known.