Objective: to describe the prevalence and severity of dementia, depression, behavioural problems and relevant medication use in a representative sample of residential and nursing care home residents. Design/setting: a survey conducted in 15 randomly selected South East London care homes. Consensus clinical dementia diagnoses were made from multi-source information, and the Clinical Dementia Rating (CDR) Scale applied. Depression was ascertained using the Cornell Depression in Dementia Scale and psychological/behavioural problems using the Neuropsychiatric Inventory (NPI). Participants: three hundred and one residents with a mean (SD) age of 83.5 (9.8) and 65.8% female were included. Results: dementia (CDR 1-3) prevalence was 75.1% overall, 55.8% in residential homes, 91.0% in residential elderly mentally infirm care and 77.0% in nursing homes. Depression prevalences were 26.5, 22.0 and 29.6%, respectively, and mean (95% CI) NPI severity scores 3.99 (3.47-4.50), 6.34 (5.29-7.39) and 6.10 (5.50-6.70) with 87.3% of the sample exhibiting at least one NPI symptom. Antidepressants were prescribed in 25.6, 25.0 and 41.3%, respectively, and antipsychotics in 7.0, 34.1 and 19.1%. Conclusion: dementia is substantially more common in care homes than recorded diagnoses would suggest, but studies using brief screening instruments may overestimate prevalence. High prevalences of depressive and/or behavioural symptoms and psychotropic use suggest significant unmet need.