Several previous reports suggest an association between treatment of patients with interferon-alpha (IFN-alpha) and development of autoantibodies and autoimmune disease. We here summarize the experience from a group of 135 patients with midgut carcinoid tumors treated with natural leukocyte IFN-alpha or recombinant IFN-alpha (rIFN-alpha). An unusual high incidence of antimicrosomal antibodies (MsAb) or anti-thyroglobulin antibodies (TgAb) and thyroid disease manifested as hyperthyroidism, hypothyroidism or a biphasic Hashimoto-like disease was seen, with female predominance. The incidence of antinuclear antibodies (ANA) was also increased, but equally in both sexes. Antibodies to parietal cells were found in 5 cases and 4 patients with pernicious anemia were detected. Two patients developed vasculitis of leukocytoclastic type and one a syndrome resembling systemic lupus erythematosus. Some patients treated with rIFN-alpha develop anti-IFN antibodies. Such antibodies may also be autoantibodies reacting with autologous IFN-alpha. They can neutralize the biologic activity of administrated IFN preparation and cause therapeutic failure. The implications of the various autoimmune manifestations during IFN-alpha treatment are discussed.