Background: Hyponatremia is the most common electrolyte disorder, but a lack of well-characterized cohorts is hindering a full appreciation of this complex and heterogeneous disorder. Methods: During 4 months, clinical, biochemical, treatment and outcome data were collected for patients presenting with hyponatremia (serum sodium <= 130 mmol/L) to an urban university hospital. Results: Forty-three patients were included (serum sodium 126.6 +/- 3.7 mmol/L). The most common causes of hyponatremia were diuretics (n=12), syndrome of inappropriate antidiuretic hormone secretion (n=11) and heart or liver disease (n=5). Renal insufficiency was frequent (n=18, 42%), and usually represented acute kidney injury (AKI; n=14, 78%). In patients with AKI, admission serum creatinine was 271 +/- 252 mu mol/L (3.4 +/- 3.1-fold increase from baseline) and the origin was usually prerenal (12/14, 86%, fractional sodium excretion 0.54% +/- 0.38%). Of these, patients with potentially reversible causes (salt loss or sepsis, n=7) had more favorable outcomes than patients with severe underlying disease (heart or liver disease, n=5), despite similar predictions using the RIFLE criteria. Survivors recovered with fluid resuscitation only. No overly rapid correction of hyponatremia was observed. Conclusions: AKI is common in patients presenting with hyponatremia and is usually of prerenal origin. The concurrence of AKI and hyperiatremia has previously not been emphasized, but is important pathophysiologically and to plan rational management for both disorders. In this cohort, isotonic fluid replacement corrected both disorders and did not lead to overly rapid correction of hyponatremia.