Community-acquired pneumonia (CAP) is a frequent cause for hospitalization and may result in a number of different renal and electrolyte complications. The purpose of this study was to describe the incidence of hyponatremia in CAP and to analyze risk factors for its occurrence. Records were reviewed for all 342 subjects who participated in the Community-Acquired Pneumonia Standardized Order Set study, a 2-year trial of supplemental treatment tools in hospital pneumonia treatment. Hyponatremia (serum sodium concentration <136 mg/dl) was present at hospital admission in 27.9% of patients. The magnitude was generally mild, only 4.1% of patients had serum sodium <130 mEq/l. Patients with hyponatremia had greater initial heart rate (100.2 vs. 93.2 beats/min, p = 0.03), white blood cell count (15,100 vs. 12,100/mul, p < 0.0001) and pneumonia severity index class 4 or 5 (35.7 vs. 25.1% of patients, p = 0.05). Hyponatremia at admission was associated with greater risk for death and increased length of hospital stay. Hyponatremia developed during the hospitalization in 10.5% of subjects, with most cases being mild, only 2.6% of all patients having serum sodium decrease to <130 mEq/l. Patients developing hyponatremia were more likely to have end-stage renal disease and to have had initial intravenous fluids other than isotonic saline, but had similar severity of illness on admission to those without acquired hyponatremia. Hyponatremia is a common complication present at the time of admission for CAP. It is associated with more severe illness, increased mortality risk and extended hospital stays. Hyponatremia develops less frequently during the hospitalization and is unrelated to severity of illness on admission, but is an iatrogenic complication and thus initial treatment with isotonic saline may reduce the risk of this complication. Copyright 2007 S. Karger AG, Basel.