Several lines of evidence support the presence of DNA damage in somatic cells of Parkinson's disease (PD) patients due to the formation of free radical species. In order to detect spontaneous chromosome and primary or oxidative DNA damage, we performed the human lymphocyte micronucleus assay (HLMNA) and comet assay in 19 PD patients and 16 healthy controls. Compared with controls, PD patients showed a significant increase in: (I) spontaneous micronucleus (MN) frequency (p<0.001); (2) single strand break (SSB) levels (p<0.001); and (3) oxidized purine base levels (p<0.05). The chromosome damage and the increased levels of oxidized purine bases observed in our patients support the hypothesis of oxidative stress as a relevant factor in the pathogenesis of PD.