Familial hypocomplementemia of the third component of complement (C3) was found in four members of a family. The prospositus had cutaneous vasculitis, hypocomplementemia, arthralgia, proteinuria and thrombocytopenia. The combination of clinical, laboratory and pathologic findings resembled the "hypocomplementemic cutaneous vasculitis syndrome" (HCVS) or the "SLE-like syndrome" but serum C3 concentration was 35 to 57 per cent of normal in the propositus and in three relatives. Results of Clq precipitins, cryoglobulins and serologic tests for systemic lupus erythematosus were negative. Proteinuria (815 mg/day) but no hematuria was present. Analysis of the C3 phenotypes in this family showed that three hypocomplementemic members were apparent homozygous C3 slow but one was heterozygous C3 fast-slow. Metabolic studies with 125-Iodinated C3 in the clinically normal mother showed a 50 per cent reduction in C3 synthesis which was consistent with hypocomplementemia documented by serum protein assay. The occurrence of an immune complex-like disease (with characteristics of the HCVS) in a patient with a familial deficiency of C3 suggests that the preexisting C3 deficiency may predispose such persons to certain diseases.