It has been well known in Japan that fasting therapy has an excellent effect on various kinds of psychosomatic diseases. From 1967 to 1977 the therapy was carried out on about 382 cases of psychosomatic diseases in our clinic with an efficacy rate of 87%. The following diseases were considered as suitable indications for the therapy; irritable colon, behavioral disturbance of eating, functional disorders of the G-I tract, neurocirculatory asthenia, labile hypertension, variable psychosomatic symptoms of puberty, conversion hysteria and many kinds of neurosis with somatic complaints. The mechanism of effectiveness of the therapy is discussed from the psychological and biological aspects. It is concluded that the regulating mechanisms of the autonomic nervous system and endocrine system may change the whole functioning of the body including the brain, resulting in a spontaneous deconditioning of maladaptive bodily and mental behavior and an improved homeostatic adjustment to life.