Neuropathological changes in Parkinson's disease progress slowly and spread according to a characteristic pattern. Recent papers have shed light on this progression of pathology by examining the fate of neurons grafted into the brains of patients with Parkinson's disease. Two of these studies demonstrate that grafted healthy neurons can gradually develop the same pathology as host neurons in the diseased brains. According to these studies, implanted neurons developed α-synuclein- and ubiquitin-positive Lewy bodies more than a decade after transplantation. We discuss the possible underlying mechanisms and their implications for how pathology spreads in Parkinson's disease.