Myeloperoxidase (MPO) generates hypochlorous acid and other reactive oxygen intermediates leading to tissue damage. MPO is expressed in macrophages-microglia in multiple sclerosis (MS) lesions. A G-->A substitution that abolishes an SP1 transcription factor consensus sequence in the promoter reduces gene expression. We studied the association of the genetic variant with MS. We did not find an association with gender, age at onset, susceptibility to, or the course and severity of MS in a population-based sample of 122 patients from Olmsted County.