To evaluate the contribution of HLA-DM alleles to susceptibility to systemic lupus erythematosus (SLE) in a Caucasian population.HLA-DMA and DMB alleles were studied in 73 patients with SLE, 147 randomly selected controls, and 86 HLA-DRB1 genotype matched controls by oligotyping of polymerase chain reaction amplified genomic DNA with sequence-specific oligonucleotide probes.There was a significant presence of HLA-DMA*0103, DMA*0104, and DMB*0102 in the SLE patients compared with the randomly selected controls. After stratification of patients and matched controls according to DRB1 genotypes, only HLA-DMA*0104 was increased in SLE patients negative for the SLE susceptibility HLA-DR alleles. For the patients and controls positive for HLA-DR allele-susceptibility for SLE, HLA-DMA*0103, DMA*0104, DMB*0102, and DMB*0103 alleles tended to be more frequent, but without reaching statistical significance. No correlation was found between HLA-DM phenotype frequencies and any clinical or biological manifestations of SLE.This is the first study evaluating the influence of HLA-DM in a Caucasian SLE population. Our results suggest that HLA-DMA*0104 may represent a novel allele of susceptibility to SLE.