The Helsinki Aging Study is based on a random sample of 795 subjects aged 75 years (N = 274), 80 years (N = 266) and 85 years (N = 255). Ninety-three demented patients were found. All were assessed for severity of dementia by Clinical Dementia Rating (CDR) scale by a general practitioner and according to the DSM-III-R criteria by a neurologist. The Mini-Mental State Examination (MMSE) was carried out by a community nurse and the Index of ADL and the IADL-scale by a close informant. The correlation of the severity of dementia between the DSM-III-R criteria and the CDR scale was moderate. The overall agreement was 64.5° and the Kappa index 0.56. The CDR scale tended to put patients in milder categories than the DSM-III-R criteria. The correlation between the clinical scales and categorized MMSE was moderate to fair. The overall agreement between MMSE and DSM-III-R criteria was 64% (Kappa 0.44) and between MMSE and CDR scale 55% (Kappa 0.33%). The dispersion of the functional scales (ADL, IADL) was much greater indicating that there were also other factors influencing the functional capacity than the degree of dementia. Different methods in staging dementia give different results thus influencing for instance the results of epidemiological studies. Functional scales are needed in clinical practice in addition to the assessment of the severity of dementia. The CDR scale is useful in assessing the need for support services.