OBJECTIVE To determine the frequency, etiology, and clinical association of hyponatremia in patients with the acquired immunodeficiency syndrome (AIDS) and AIDS-related complex (ARC). METHODS A prospective analysis of 167 patients with AIDS and 45 patients with ARC admitted on 259 occasions to a large metropolitan teaching hospital during a 3-month period. RESULTS Eighty-three patients (39%) with hyponatremia (serum sodium concentration less than 135 mmol/L) were observed during 99 hospitalizations, for a frequency of 38%. The mean (+/- standard error) of the lowest serum sodium concentration was 128 +/- 1 mmol/L in the hyponatremic patients and 138 +/- 1 mmol/L in the normonatremic patients. Hyponatremia was present on admission during 57 hospitalizations and was associated with gastrointestinal losses and hypovolemia in 43%. When hyponatremia developed during hospitalization, 68% of the patients were clinically euvolemic and had a syndrome consistent with inappropriate secretion of antidiuretic hormone (SIADH). Patients with hyponatremia were hospitalized longer than those with normal serum sodium concentrations (17 +/- 1 versus 9 +/- 1 days, p < 0.001). In addition, the mortality rate in the hyponatremic group was higher than that in the normonatremic group (36.5% versus 19.7%, p < 0.01). CONCLUSIONS Hyponatremia is a common electrolyte disorder in patients hospitalized with AIDS or ARC and is frequently associated with gastrointestinal losses or SIADH as well as increased morbidity and mortality.