The cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4; CD 152) is a negative regulator of T-lymphocyte activation. Particular genotypes of the locus encoding the CTLA-4 glycoprotein have been associated with susceptibility to various autoimmune diseases. To determine their role in susceptibility to systemic lupus erythematosus (SLE), we genotyped 130 patients and 200 ethnically matched controls for allelic determinants at four polymorphic sites, viz., three in the promoter region at positions -1722 (T/C), -1661 (A/G), and -318 (C/T), and one within the first exon at position +49 (A/G), by restriction fragment length polymorphism methods. All genotype frequencies were in Hardy-Weinberg equilibrium. The genotypes at position -1722 were significantly associated with SLE. The frequency of T/T homozygotes was higher in patients than in controls (56% vs 33%, P=0.00003). Conversely, the frequencies of C/C homozygotes and C/T heterozygotes were higher in controls than in patients (15.5% vs 7%, P=0.019; 51.5% vs 37%, P=0.009). Genotypes at positions +49, -318, or -1661 were not significantly associated with SLE. These results show that allelic variation at the -1722 site influences susceptibility to SLE. This is the first report to our knowledge implicating CTLA-4 genotypes at the -1722 locus in susceptibility to any disease.