Results of clinical, serologic and histologic studies documenting an association between hereditary C6 deficiency and a connective tissue disease are provided. The propositus had systemic lupus erythematosus with prominent discoid features, Sjögren's syndrome and hyperthyroidism. Serum C6 was undetectable by radial immunodiffusion and hemolytic assays. Serologic and typing studies performed on 9 family members suggested an autosomal codominant transmission. No correlation with a specific HLA phenotype was established.