Two siblings (case 1 and case 2) with homozygous C1q deficiency are described. Both presented with a photosensitive rash, and during follow-up case one developed SLE with nephrotic range proteinuria. Case 2 had microscopic hematuria with a past history of macroscopic hematuria. Renal biopsies revealed mesangioproliferative glomerulonephritis in case 1 and IgA nephropathy in case 2, a new finding in association with C1q deficiency. Since the classical pathway of complement plays a role in the development of antibody responses, the family was also evaluated for the immune response to hepatitis B vaccine. Antibody response to hepatitis B vaccine was normal in both affected members and the rest of the family. The A-, B- and C- chain genes of C1q were amplified by PCR and directly sequenced. A homozygous C to T point mutation was identified in genomic DNA isolated from the patients at codon 186 in the A chain that resulted in a premature stop codon. This mutation was present in both parents and both unaffected sibs in the heterozygous state. This mutation was identical to that previously described in a Slovakian family with C1q deficiency. Because of this finding, a series of 92 genomic DNA samples was screened from ethnically distinct patient groups with SLE to test the hypothesis that this mutation of C1q may be a widespread disease susceptibility gene. No further examples of this mutation were found.