The authors of this large cohort study show that one-third of older people with delirium in hospital will develop dementia within 5 years.

In the longest and largest cohort study of delirium patients to date Leighton et al 1 report that one-third of people aged over 65 diagnosed with delirium in hospital will develop dementia, and another half will be dead, within 5 years.

Inherent in the use of pre-existing data to establish a cohort of participants without a diagnosis of dementia, especially as far back as 1996, is the risk of including people with undiagnosed dementia. It is likely that a sizeable proportion of older people diagnosed with delirium in hospital has undiagnosed dementia.2 This is supported by the fact that 9% were diagnosed with dementia in the first 6 months after their hospital-based delirium diagnosis, which implies a likely dementia at the time of their delirium.

The second …