We have shown that the majority of elderly female patients in continuing care are in a hyperosmolar state. Thirty-two out of thirty-nine subjects had an osmolality greater than 295 mosm/kg and the mean osmolality was 302 ± 8 mosm/kg (range 280–317 mosm/kg). No significant relationship was found with the degree of confusion (rs 0.05). In the second part of the study neither the measured degree of hydration, or any acute, small increases in fluid input influenced the plasma osmolali