Disorders of serum proteins, circulating antibodies, immune complexes to the dermo-epidermal junction, B- and T-cell markers from peripheral blood lymphocytes and the suppressor function of peripheral mononuclear cells have been investigated in 106 patients with histologically proven discoid lupus erythematosus (discoid LE). The results confirm that this condition is primarily a localized skin disease with low concentrations of autoantigens and circulating autoantibodies. By contrast, systemic lupus erythematosus (systemic LE) is a multifocal, generalized disease with high concentrations of autoantigens, autoantibodies and immune complexes. The transformation of discoid LE to systemic LE is discussed, and a possible two-hit mechanism is proposed.