The causes, therapy for, and consequences of hypernatremia in elderly patients are not well understood. We found that in 15 187 consecutive hospital admissions, 162 (1.1%) patients who were at least 60 years of age had serum sodium levels that measured greater than 148 meq/L. Of that 162, 57% had become hypernatremic in the hospital; the remaining 43% were hypernatremic at hospitalization. The mean peak serum sodium level was 154 meq/L (range, 149 to 182), and mean water deficit, 9% of total body water (range, 6% to 30%). The most frequent primary causes were complications of surgery (21%), febrile illness (20%), infirmity (11%), and diabetes mellitus (11%), with more than 40 causal factors identified. Depression of sensorium correlated with severity of hypernatremia (p < 0.001). The mortality rate (42%) was seven times that of age-matched hospitalized patients, but was not predicted by severity of hypernatremia. Mortality increased with increasing rates of fluid replacement (p < 0.008). Hypernatremia in elderly patients is usually iatrogenic and often a marker for severe associated systemic illness.