To obtain accurate estimates of the prevalence of age-associated memory impairment, dementia, and Alzheimer's disease, a population study was carried out in Turégano, a rural community of 1011 inhabitants in the Segovia province of Spain. The study was divided into two phases: a door to door survey of the entire population aged 40 years and over (503 persons), followed by a clinical examination of suspected cases for positive and differential diagnosis of dementia and cognitive impairment. The prevalence of age-associated memory impairment was 3.6% in individuals of 40 years and over and 7.1% in individuals of 65 years and over, whereas dementia was found in 2.6% and 5.2%, respectively. The prevalence rates of both clinical conditions increased with age. The most prevalent clinical category of dementia was dementia of Alzheimer type, which represented 1.8% and 3.8% of these two age groups. The corresponding figures for vascular dementia were 0.4% and 0.9% and for secondary dementia 0.4% and 0.5%. Age-associated memory impairment is an age-dependent disorder with a high prevalence among the elderly; some of these patients may represent an early stage of Alzheimer's disease, suggesting that the prevalence of this disorder may be higher than previously estimated.