The ecogenetic theory contends that most cases of Parkinson's disease (PD) result from the actions of environmental factors in genetically susceptible individuals on a background of normal ageing. This notion is supported by epidemiologic data; family history of PD and exposures to environmental toxins such as pesticides increase risk, while cigarette smoking reduces risk. As a result, polymorphic genes that code for metabolic enzymes have been considered as candidates for conferring differential risk for PD. Given their prominence in xenobiotic metabolism, the cytochrome P450 (CYP) genes have come under great scrutiny. The activity of CYP2D6 is largely determined by genetic variability and common sequence variants exist in human populations that lead to poor metaboliser (PM) phenotypes. These have been extensively studied as genetic risk factors for PD with inconsistent results. However, these studies have disregarded interactive effects (e.g. gene x environment interactions) despite the assertions of the ecogenetic theory. Data from our group and others suggest that the CYP2D6 PM genotype interacts with certain environmental factors such as pesticide exposure and cigarette smoking to confer differential risk for PD. Previous failure to consider such interactions might, in part, explain the inconsistencies observed in the CYP2D6 genetic risk-factor literature. Our data illustrate, using CYP2D6 as an exemplar, that it is crucial to consider both genetic and environmental factors, and their interactions, in any examination of risk factors for PD.