The -330 IL2 gene promoter polymorphism has been associated with multiple sclerosis (MS) [J. Neuroimmunol. 119 (2001) 101], but the basis underlying this association remains unknown to date. In the present work, we have found that IL2 promoter-luciferase constructs, transfected in Jurkat cell line, showed twofold higher levels of gene expression in the -330 G allele. However, the transcriptional effect of this polymorphism in lymphocytes showed that the G allele was related to lower expression of IL2. This difference increased in the patient group. Divergence between in vivo and in vitro influence of the -330 IL2 promoter polymorphic site suggests the existence of additional unknown polymorphisms affecting gene regulation. Our data show an increased IL2 expression among GT and TT genotypes previously associated with susceptibility to MS.