In the UK the recent House of Lords ruling on the Bournewood case provided a statutory basis for admitting patients into psychiatric beds who lack the capacity to consent, but do not dissent, without recourse to detention under the Mental Health Act. A study to ascertain the prevalence and correlates of the lack of capacity to consent to geriatric psychiatry inpatient admission was undertaken. All consecutive acute inpatient admissions to a geriatric psychiatry unit over a six-month period were examined by an independent research psychiatrist (SM). Data on demography, diagnosis, severity of cognitive impairment, insight, the SM's assessment of capacity to consent to the admission and the consultant psychiatrists' assessment of the capacity to consent to the admission were ascertained. The overall prevalence of lack of capacity to consent to geriatric psychiatry inpatient admission was 48%. It was associated with a diagnosis of dementia, increased severity of cognitive impairment, reduced insight and detention under the Mental Health Act. The kappa concordance between SM's and the consultant psychiatrist's assessment of capacity was modest at 0.6. These findings require replication in a larger multi-centre study, perhaps using standardised instruments to measure capacity. A large number of psychiatric patients are informally admitted despite lacking the capacity to consent to the admission because they do not dissent. These patients do not enjoy the safeguards available under the Mental Health Act.