Parkinson's disease is a multifactorial disease. In early-onset cases, genetic predisposition may play a role, but typical Parkinson's disease is most likely determined by genetic and environmental factors. Since age is a consistent risk factor, an age-dependent cumulative insult mechanism may be responsible for the selective degeneration of nigrostriatal neurons. Although there is no cure, several types of treatments are currently available to substantially reduce the clinical symptoms of Parkinson's disease. An understanding of the age-dependent complex pattern of genetic variation (SNP genotyping) together with the genetic expression profile (using chip technology) of the degenerating neurons and the effect of environmental factors is a prerequisite to deciphering the mechanism of pathogenesis. This understanding may lead to predicting drug response and ultimately to developing preventive medicine at the individual level in the future.