Both natural and recombinant interferons (IFNs) exert antitumor effects in man. Much work has to be done to construct optimal use of the IFN system. Its physiological role is not fully understood. Various IFN effects are described, all of which probably play a role for clinical effectiveness. Doses and schedules are discussed and pharmacokinetics and side-effects described. IFN therapy of benign and malignant tumor disease is pursued as clinical trials. Efficacy on various diseases is reported in the present article. Combination therapy is suggested for the future.