The cytotoxic T-lymphocyte antigen 4 (CTLA4) is an important modifier of T-cell activation with down-regulatory properties upon B7 engagement. We investigated the association of the CTLA4 A/G dimorphism in exon 1 (+49) with disease susceptibility, disease course and severity. No differences in the allelic distribution of the G(49) allele between multiple sclerosis (MS) patients and the control group was found. However, the G(49) allele occurred in a significant higher percentage of patients with primary progressive MS compared to patients with bout onset of disease. The results suggest that dysregulation of CTLA4-driven down-regulation of T-cell function due a genetic dimorphism in exon 1 may be involved in the pathogenesis of different MS disease subtypes.