As evidence emerges that complex gene alterations are involved in the onset of Parkinson's disease (PD), the role of environmental chemicals in the pathogenesis of this disease becomes intensely debated. Although it is undisputed that acute exposure to certain chemicals such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is sufficient to cause human parkinsonism, the evidence that the risk for PD increases because of environmental exposure is generally weaker. Several studies have suggested that pesticide exposure and life in rural areas are significant risks factors for PD. Among other pesticides, paraquat (PQ) has been linked to PD by epidemiological studies and experimental work in rodents, in which it causes lesions in the substantia nigra, pars compacta. However, the evidence that human exposure to the chemical results in an increased risk for PD is rather limited and based on insufficient epidemiological data. This review critically analyses the evidence that implicates PQ in parkinsonism and discusses the limitations of chemical modelling of PD.