A case of systemic lupus erythematosus associated with an homozygous deficiency in the second fraction of complement is reported and compared to previous reports of the literature. The high incidence of infections in these patients is outlined. The defective gene in this family was associated with the HLA A10B18 haplotype and the propositus was homozygous at the HLA-D locus. Familial study allowed the detection of 3 heterozygous individuals two of them being symptomatic (vascular purpura, high incidence of bacterial infections).