Etiology of sporadic Parkinson's disease (PD) is largely unknown. The contribution of genetic factors to the pathogenesis of PD is supported by the demonstration of high concordance in twins, increased risk among relatives of PD patients and existence of familial cases. This study aimed to examine the relation between interleukin 18 (IL-18) gene promoter polymorphisms and idiopathic PD, and its impact on clinical presentation and disease severity. 30 idiopathic PD patients and 15 age- and sex-matched healthy subjects were included. Disease severity was assessed using Unified Parkinson's Disease Rating Scale (UPDRS). Genetic testing for IL-18 gene promoter -607C/A single nucleotide polymorphisms (SNP) was done using real-time polymerase chain reaction (PCR) technique. A raised risk of PD development was found in patients with A/C and C/C genotypes of the site -607C/A (odds ratios = 1.83 and 1.98, respectively). The distribution of the genotypes showed no significant relation to gender or predominant clinical presentation. The age at onset of disease was significantly lower in C/C and A/A genotypes compared to A/C genotype (p = 0.001 and 0.04, respectively). Patients with A/A genotype showed significantly higher mentation score of UPDRS compared to patients with A/C and C/C genotypes (p = 0.003 and p = 0.002, respectively). Polymorphisms of IL-18 gene promoter increase the risk of developing idiopathic PD. The polymorphisms may affect phenotypic expression rather than being a direct cause of idiopathic PD.