1. The clinical, experimental, and theoretical literature on carbon dioxide oxygen inhalation therapy (CDT) is reviewed and critically evaluated.2. The over-all improvement rate in 238 nonpsychotic patients treated with CDT, collected from the literature, is 61%. The highest rate of improvement (83%) is reported in psychophysiologic disorders, the lowest (46%) in stutterers. The evidence indicates that these high improvement rates are exaggerated since most workers include "minimal" improvement in their statistics. If these patients had been correctly classified "unimproved," the rates would be lower and much more accurate.3. CDT is apparently of little value in the treatment of psychoses, severe obsessive-compulsive, and hypochondriacal reactions.4. CDT may be a useful adjunct to psychotherapy, causing rapid release of unconscious repressed memories and affects (abreaction), thereby shortening the duration of psychotherapy.5. CDT is capable of causing amelioration of some neurotic symptoms without the use of concurrent psychotherapy. Ideally, however, CDT should be combined with or followed by psychotherapy in order to induce lasting improvement in the patient through the development of insight into the cause of his symptoms and emotional difficulties.6. No fatalities or lasting complications from CDT have been reported. It is apparently the least harmful form of somatic therapy in psychiatry. However, since CDT is "potentially" dangerous both physically and psychologically, it should be given only by psychiatrists or other physicians with special psychiatric training. Indiscriminate use by untrained personnel might result in serious complications.7. CDT may be of value in certain patients who are hostile to psychotherapy or who for various reasons are unable to receive other forms of treatment. Such individuals frequently become more cooperative during CDT, since they feel that something "active" is being done to help them and they frequently request further psychiatric treatment.8. A major difficulty of CDT is the increasing severe anxiety that it induces, which causes many patients to discontinue treatment prematurely.9. The mode of action of CDT is unknown. There is no scientific evidence that CO2 is a specific agent in the treatment of nonpsychotic mental disorders, since many other anaesthetic agents exert similar effects.10. Suggestions for further research are made, with particular emphasis on the need for adequate controls, prolonged follow-up studies, and physiological and biochemical analyses of the effects of CDT on man and experimental animals.