In 1981 a survey of elderly persons aged 75 years and over who belonged to a general practice in Melton Mowbray, Leicestershire found a prevalence of moderate cognitive impairment of 4.7%. The criterion which determined the impairment was a score of 7 or under on the CAPE Information/Orientation (IO) sub-test. The prevalence rate has been considered to be much lower than in some key studies, notably the 13% reported in 1970 from Newcastle upon Tyne. Although rates based on cognitive scales are likely to give different results from those based on diagnostic assessment, the suggestion is that the IO sub-test is an insensitive screening instrument for dementia. Using results on the sensitivity and specificity of the CAPE IO sub-test (cut-point 8/9) to detect moderate or severe dementia as defined by clinical diagnosis using the Cambridge Mental Disorders of the Elderly (CAMDEX) schedule, the adjusted prevalence rates of moderate or severe dementia were found to be 3.4% in 1981 and 5.2% in 1988. Both these figures were lower than the observed rates scoring 8 or under on the IO sub-test, confirming that insensitivity of the IO sub-test was not the reason for the supposed low rate in Melton Mowbray in 1981. As the prevalence of dementia in those aged 75 years and over has been reported by other studies to range from 3% to 24%, it is more likely that high rates are due to screening instruments with low specificity.