BACKGROUND: The number of older adults with dementia will increase around the world in the decades ahead as populations age. Current estimates suggest that about 4.2 million adults in the US have dementia and that the attributable economic cost of their care is about $200 billion per year. The worldwide dementia prevalence is estimated at 44.3 million people and the total cost at $604 billion per year. It is expected that the worldwide prevalence will triple to 135.5 million by 2050. However, a number of recent population-based studies from countries around the world suggest that the age-specific risk of dementia may be declining, which could help moderate the expected increase in dementia cases that will accompany the growing number of older adults. DISCUSSION: At least nine recent population-based studies of dementia incidence or prevalence have shown a declining age-specific risk in the US, England, The Netherlands, Sweden, and Denmark. A number of factors, especially rising levels of education and more aggressive treatment of key cardiovascular risk factors such as hypertension and hypercholesterolemia, may be leading to improving 'brain health' and declining age-specific risk of Alzheimer's disease and dementia in countries around the world. SUMMARY: Multiple epidemiological studies from around the world suggest an optimistic trend of declining population dementia risk in high-income countries over the past 25 years. Rising levels of education and more widespread and successful treatment of key cardiovascular risk factors may be the driving factors accounting for this decline in dementia risk. Whether this optimistic trend will continue in the face of rising worldwide levels of obesity and diabetes and whether this trend is also occurring in low- and middle-income countries are key unanswered questions which will have enormous implications for the extent of the future worldwide impact of Alzheimer's disease and dementia on patients, families, and societies in the decades ahead.