Plaques formed from amyloid-beta protein were discovered in the grey matter and blood vessels.
Fears are growing that Alzheimer's may be passed from person to person after researchers reported a second case suggesting the disease could be transmitted by 'medical accidents'.
Swiss doctors found signs of Alzheimer's in the brains of people who died of the rare, brain-wasting Creutzfeldt-Jakob disease (CJD).
Decades before their deaths, the individuals had all received surgical grafts of dura mater, the membrane that covers the brain and spinal cord, from human bodies.
These grafts were contaminated with the prion protein that causes CJD.
In addition to the damage caused by the prions, five of the brains displayed some of the pathological signs that are associated with Alzheimer's disease.
Plaques formed from amyloid-beta protein were discovered in the grey matter and blood vessels.
The individuals, aged between 28 and 63, were unusually young to have developed such plaques.
Researchers writing in Swiss Medical Weekly suggest the transplanted dura mater was contaminated with small 'seeds' of amyloid-beta protein - which some scientists think could be a trigger for Alzheimer's.
The researchers from Austria and Switzerland said the find was "highly unusual and suggests a causal relationship to the dural grafts".
They added "further studies" would be needed and called for a "critical re-evaluation" of how surgical instruments were cleaned.
In September, Professor John Collinge, director of the Medical Research Council Prion Unit at University College London, first discovered evidence the amyloid beta proteins could potentially be transferred from one person to another during some medical procedures.
Speaking of the latest study he said we "need to rethink our view of Alzheimer's and evaluate the risk of it being transmitted inadvertently to patients".
He told Nature: "Our results are all consistent. He fact that the new study shows the same pathology emerging after a completely different procedure increases our concern."
But Britain's top doctor, Prof Dame Sally Davies, was quick to reassure the public after the controversial findings were published in medical journal Nature.