Objectives This paper aimed to measure the prevalence and outcomes of delirium for patients over 70 admitted to a general hospital for acute medical care and to assess the validity of the Delirium Rating Scale‐Revised‐98 (DRS‐R‐98) in this setting. Methods Prospective study in a British acute general hospital providing sole emergency medical services for its locality. We screened consecutive patients over 70 with an unplanned emergency hospital admission and recruited a cohort of 249 patients likely to have mental health problems. They were assessed for health status at baseline and followed over 6 months. A sub‐sample of 93 participants was assessed clinically for delirium. Results 27% (95% confidence interval (CI) 23–31) of all older medical patients admitted to hospital had DRS‐diagnosed delirium, and 41% (95% CI 37–45) had dementia (including 19% with co‐morbid delirium and dementia). Compared with clinician diagnosis, DRS‐R‐98 sensitivity was at least 0.75, specificity 0.71. Compared with reversible cognitive impairment, sensitivity was at least 0.50, specificity 0.67. DRS‐diagnosed delirium was associated with cognitive impairment, mood, behavioural and psychological symptoms, activities of daily living, and number of drugs prescribed, supporting construct validity. Of those with DRS‐diagnosed delirium, 37% died within 6 months (relative risk 1.4, 95% CI 0.97–2.2), 43% had reversible cognitive impairment, but only 25% had clinically important recovery in activities of daily living. Behavioural and psychological symptoms were common and mostly resolved, but new symptoms frequently developed. Conclusion Delirium is common. Some, but not all, features are reversible. DRS‐R‐98 has reasonable validity in populations where co‐morbid dementia is prevalent. Copyright © 2013 John Wiley & Sons, Ltd.