Free-radical-mediated mechanisms may contribute to neuronal damage in Parkinson's disease (PD), other neurodegenerative conditions also associated with aging, and the aging process itself. Cytotoxic free radicals are generated in the brain by oxidation/reduction reactions that are catalyzed by transition metals such as iron. Any regional increase in brain iron concentration may increase the potential for local free-radical formation. The purpose of this study was to determine the relationship between age and basal ganglia iron content in 20 normal individuals ranging from 24 to 79 years of age. We used an in vivo magnetic resonance method to quantify the effects of paramagnetic centers sequestered inside cellular membranes, thereby enabling the determination of a quantitative index of local brain iron content. We observed a strong direct relationship between age and regional iron content in the putamen (r = 0.76, p < 0.0001) and caudate (r = 0.69, p < 0.001), but not in the globus pallidus (r = 0.32, p = 0.17) or thalamus (r = 0.13, p = 0.58). In conclusion, striatal iron content increases with advancing age. This increase may increase the probability of free-radical formation in the striatum, therefore representing a risk factor for the development of neurodegenerative disorders such as PD in which nigrostriatal neurons may be affected by increased oxidant stress.