Three patients from two families with complete hereditary deficiency of the fourth component of complement (C4) and systemic lupus erythematosus are described. The syndrome presented by these patients is characterized by early onset in life; exquisite sensitivity to sunlight and to cold exposure, the latter resulting in Raynaud's phenomenon; and skin lesions involving not only exposed areas of the body but also palms and soles and presenting as butterfly rashes, maculopapular eruptions, and lesions similar to those of chronic discoid lupus erythematosus, with marked scaling, atrophy, and scarring. Lupus erythematosus (LE) cell tests were negative and antinuclear antibody (ANA) titers low or negative. The male patient of our series died at the age of 3½ years from septicemia, whereas the two girls, aged 18 and 11 years, respectively, were alive at the time of writing. The C4-deficient gene is associated with HLA-Aw32, Bw38, and Bf S in one family and with HLA-A30, B18, DR7, and Bf S₁ in the other family; the latter is the second family in which this HLA haplotype has been found to be associated with hereditary C4 deficiency.