Human ICOS is a T cell costimulatory molecule supporting IL10 secretion. A pilot study investigating variations of the ICOS gene 3'UTR detected 8 polymorphisms forming three haplotypes (A, B, C). Haplotype-A and -C displayed the highest difference. Activated T cells from healthy AA homozygotes expressed significantly less ICOS and secreted more IL10 than AC heterozygotes, whereas AB heterozygotes displayed intermediate levels. Analysis of 441 multiple sclerosis patients and 793 controls showed that frequency of AA homozygosity was significantly lower in MS patients with relapsing-remitting onset (N=416) than in controls (OR=0.70). Moreover, AA patients with relapsing-remitting onset had lower relapse rate and multiple sclerosis severity score than non-AA patients.