In view of the potential importance of the proteolytic mechanisms in the evolution of MS lesions, the authors studied the 4G/5G promoter polymorphism of the plasminogen activator inhibitor 1 (PAI-1) gene in the susceptibility of MS. The 5G5G genotype was associated with MS in women at an odds ratio of 2.3 (95% confidence interval, 1.04 to 5.23). The genotype seems to be a low producer of PAI-1, suggesting that reduced capacity for proteinase inhibition may be involved in the etiopathogenesis of MS in women.