In a prospective study hyponatremia was observed in 30 per cent of 100 elderly hospital in-patients, the serum sodium ranging from 106 to 129 mmol/l. In 67 per cent of the subjects symptoms were absent or non-specific, and the remainder had symptoms of altered sensorium without any focal neurological deficit. Hyponatremia was iatrogenic in 30 per cent, while 17 per cent had an underlying bacterial chest infection. Removal of the cause rapidly reversed hyponatremia and the relevant symptoms. It is stressed that early detection and reversal of this abnormality would significantly limit morbidity in the elderly.