MRI is a relatively simple, safe, in vivo technique that has proved to be of great value in differentiating patients with Parkinson's disease from those with atypical parkinsonism, and it is the first laboratory marker to be able to do so. By permitting the in vivo detection of increased iron in the nigra and striatum of patients with parkinsonism, MRI may also have provided a clue into the mechanism of cell death in neurodegenerative disorders. Finally it is conceivable that MRI scanning may serve as a screening tool that permits the early recognition of pathologic iron accumulation before the development of clinical symptoms. This is reasonable to anticipate for patients with atypical parkinsonism in whom large quantities of iron appear to accumulate in the putamen early in the disease. It is possible that with some refinement, abnormalities in the nigra may be detected on MRI with a higher level of certainty. In an era in which we may be able to provide neuroprotective therapy, MRI might be an important technique for defining a population of patients at risk for the development of Parkinson's disease who might benefit from presymptomatic therapy.