A pedigree of C1 inhibitor (C1 INH) deficiency associated with positive LE cell and an elevated titer of DNA antibodies and antinuclear factor (ANF) and nephropathy was presented. The proband of this family was diagnosed as having definite systemic lupus erythematosus (SLE) after a clinical course of several year since her first visit to our hospital and because of the lack of hemolytic activity of complement (CH50), in spite of the absence of idiopathic edema, C1 INH levels were 1.2 mg/dl (3.8% NHS) determined as antigen and 65 site forming unit (SFU) (5.8% NHS) determined by hemolytic assay in her blood. Her mother and brother had characteristic idiopathic edema of her face, larynx, hand and bowel and they had low levels of C1 INH of 1.8 mg/dl (5.8% NHS) and 4.8 mg/dl (15.5% NHS) respectively in their blood. On the basis of these findings, this family was diagnosed as having a pedigree of hereditary angioneurotic edema (HANE) which is supposedly an inherited autosomal positive trait. Actually, however, the proband's serological and hematological indices became positive and progressed year by year, which implies that SLE was absent for the first several years. It might be said that this interesting clinical course indicates that SLE appeared chronologically as a hereditary deficiency in one of the complement components in this case. In concurrence with the general observation that recurrent viral infections due to the deficiency of complement components are presumed to be responsible for SLE-like disease. Her levels of several kinds of anti-virus antibodies were high. Methylprednisolone helped normalize her level of C1 INH and ameliorate the clinical course.