A new study suggests that Alzheimer's related changes can be traced 25 years before memory and thinking problems appear.
This may offer a valuable guide for companies looking to test new treatments in people at an earlier stage.
The study in the New England Journal of Medicine offers a timeline of changes in spinal fluid, brain size and the appearance of brain plaques.
The research determines that there are also other factors that precede the onset of Alzheimer's in people who are genetically predestined to develop the brain-wasting disease.
Dr Randall Bateman of Washington University School of Medicine in St Louis, Missouri, who helped lead the study, said "it is really the first report that we have in living people of these changes."
Current drugs for Alzheimer's only treat symptoms.
No drugs have yet been able to keep the fatal disease from progressing.
Some researchers think that maybe the inability to stop the disease from progressing is because the disease has been studied too late in its progression.
The study focuses on families enrolled in the Dominantly Inherited Alzheimer's Network, a consortium that enrols people who are genetically predisposed to develop the disease at an early age.
People in these families have a 50% chance of inheriting one of three genes that cause early Alzheimer's.
Most develop symptoms around the same time as their affected parent.
On average, patients with this form get Alzheimer's disease by 45.