Systemic lupus erythematosus (SLE) is a rare disease in childhood. Here two cases with SLE are presented, both with C4 null alleles yielding a functional C4 deficiency. The first case, a 14-year-old girl with a C4A null allele only, had a mild disease course, whereas the second child, a seven-year-old boy with both C4A0 and C4B0, had a more relentless course leading to death in five years. We conclude that complement activation by the classical pathway might be an essential mechanism that protects against the emergence of autoimmune or immune-complex diseases, and that the deficient state in our patients predisposed them to the early development of SLE.