A thousand patients were studied in order to determine what special hazards are associated with anesthetization in obesity. Tests of pulmonary ventilation were emphasized. As a result, hazards were classified as either mechanical or physiological. In general anesthesia by inhalation, mechanical difficulties with breathing in the obese patient usually led to clinical signs of hypercapnia and hypoxia in addition to the preexisting circulatory and metabolic handicaps. In cases of administrating anesthesia by intravenous methods, the most common hazards were associated with venipuncture, laryngospasm, and overdosage. In spinal anesthesia, illustrated by three case histories, lumbar puncture was more difficult, the evaluation of pulmonary function less accurate, and the control of the level of anesthesia less satisfactory in the obese than in the nonobese patient.