Migraine is a debilitating disorder affecting approximately 12% of Caucasian populations. The disease has a large genetic component, although at present the type and number of genes involved is unclear. Candidate gene studies may be useful strategies for identifying genes involved in complex diseases such as migraine, especially if the gene being examined contributes only a minor effect to the overall phenotype. Nitric oxide (NO) is emerging as a key molecule affecting the pain associated with migraine. Since NO synthase (NOS) enzymes catalyze the synthesis of NO, the genes that code for these enzymes are good candidates for migraine molecular genetic analysis. This study investigated the role of a functionally relevant bi-allelic tetranucleotide polymorphism located in the promoter region of the human inducible nitric oxide synthase (iNOS) gene in migraine etiology. A large group of migraine affected individuals (n = 261) were genotyped and compared with an age- and sex-matched group of unaffected controls (n = 252). Results of a chi-squared analysis indicated that allele distributions for both migraine cases and controls were not significantly different (?2 = 1.93, P = 0.16). These findings offer no evidence for an allelic association of the tested iNOS polymorphism with the common forms of the disease and therefore do not support a role for this gene in migraine pathogenesis.