Thirty-three patients who were anxious, depressed, or hysterical were treated by inhalation with a mixture of 30% carbon dioxide and 70% oxygen. These patients were followed from 6 months to 2 years. Of this group, 4 patients responded promptly and sustained their improvement. Eight patients showed slight and usually temporary improvement over a period of 6 to 8 weeks. Improvement is defined as a decrease in the number and severity of the symptoms and a return to a premorbid level of adjustment. Twenty-one of the 33 patients received no appreciable help from this method of treatment and in this series this procedure was considered to be of limited value.