Hereditary complete deficiency of complement component C1q is associated with a high prevalence of systemic lupus erythematosus and increased susceptibility to severe recurrent infections. An 11-year-old girl was screened for immunodeficiency due to a history of recurrent meningitis and pneumonia. Immunologic studies revealed absence of classic pathway hemolytic activity and undetectable levels of Clq. Exon-specific amplification of genomic DNA by polymerase chain reaction followed by direct sequence analysis revealed a novel homozygous missense mutation at codon 48 in the C1q C gene causing a glycine-to-arginine substitution affecting the collagen-like region of C1q. No changes were seen in the exons of the A and B chains. The mutation affected both the formation and the secretion of C1q variant molecules. We describe a novel mutation in the C1q C chain gene that leads to an interchange in amino acids resulting in absence of C1q in serum.