Fifty-nine PD patients were subdivided into four subgroups according to the presence/absence of genetic factors (PARK1, 2 and 8) and a history of exposure to environmental factors. Age at onset of the disease was significantly lower in patients with both a genetic factor and a history of exposure as compared to those without any risk factors for PD at all (on average 62 years versus 72 years). The age at onset was intermediate in patients with either a genetic risk factor or a history of exposure. These results support the theory that PD is the result of an interaction between genetic predisposition and exposure to environmental factors.