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 less than 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 less than 0.008). Hypernatremia in elderly patients is usually iatrogenic and often a marker for severe associated systemic illness.