We describe a 27-year-old women with systemic lupus erythematosus, C1q deficiency and cytomegalovirus retinitis. She suffered from severe SLE, with cutaneous and CNS involvement, and died of CNS disease aged 28. Review of 29 other published cases of C1q deficiency shows that SLE in these patients is often severe (five with CNS disease, ten with glomerulonephritis). The results of autoantibody studies in this and another patient with C1q deficiency and SLE are presented--both patients had autoantibodies to the extractable nuclear antigens, Sm, RNP and Ro, and one patient had high titres of antibodies to dsDNA. One of the patients had previously been treated with fresh frozen plasma, and antibodies to C1q were present in his serum. Homozygous C1q deficiency is associated with a very high prevalence of severe SLE with the full panoply of autoantibodies characteristic of this disease.