Hyponatremia is a common electrolyte abnormality in AIDS patients. In this study, hyponatremia was defined as natremia less than 130 mmol/l at two different samplings. Medical records of 160 hospitalized AIDS patients were exhaustively reviewed in search for hyponatremia and, if present, of its etiology. 45 cases of hyponatremia were identified in 43 AIDS patients. Two causes were predominant: hypovolemic hyponatremia, due to water and salt losses (11 cases) and the syndrome of inappropriate antidiuretic hormone secretion (10 cases). These results are consistent with those of the literature and were used to develop a simple diagnosis schedule based on the analysis of limited clinical and biological data: hydration status, serum and urinary osmolality, natriuria and creatininemia.