Delirium (also known as ‘acute confusional state’) is common in physically ill old people, affecting up to 50 per cent of those acutely ill in hospital as well as up to 15 per cent postoperatively. In selected groups (eg, those with dementia), the prevalence may rise to nearer 90 per cent1. Professors Stephen Curran and John Wattis discuss the experience, the impact, the detection and differential diagnosis, and the practical management of delirium.