HARI SREENIVASAN:

The CDC ranks Alzheimer's as the sixth-leading killer in the U.S., accounting for nearly 85,000 deaths a year. But the study in the journal "Neurology" puts the annual death toll around half-a-million, making it the third-leading cause of death, just behind heart disease and cancer, and ahead of chronic lung disease and strokes.

Dr. Bryan James, an epidemiologist with Rush University Medical Center in Chicago, led the research. He joins us now.

So, what's responsible for this discrepancy? They say 85,000. You say half-a-million. That's a big gap.