The possibility of using magnetic resonance (MR) to evaluate the severity of the pathological changes of Parkinson's disease (PD) is suggested by the known accumulation of iron in the basal ganglia in PD and the reduced signal evident from this area with conventional T2-weighted MR imaging. To improve the specificity of MR for the measurement of tissue iron content, we have developed a method that quantifies the effects of paramagnetic centers sequestered inside cellular membranes, based on the echo time dependence of the decay of transverse magnetization caused by the local field inhomogeneities which are due to intracellular iron. This method enables an index of local tissue iron content to be calculated for structures of the basal ganglia. We report here the application of this method to a series of patients with PD (n = 12) and of normal, age-matched controls (n = 13). Our objective was to determine whether this measurement of basal ganglia iron concentration correlates with the presence and severity of PD. We observed a significant increase in iron content in both the putamen and pallidum in PD as well as a correlation with the severity of clinical symptomatology. More severely affected patients had a higher iron content in both of these structures. Our results suggest that this MR measurement may provide a noninvasive method of measuring the severity of the pathological changes underlying PD.