This paper reports a retrospective study of the preoperative and postoperative management of 28 patients who underwent thymectomy between 1956 and 1973. Patients who received postoperative artificial ventilation were compared with the group who did not with respect to sex, age, severity of disease, preoperative vital capacity, and thymic histology. Evidence is presented that postoperative artificial ventilation is required when the preoperative vital capacity with the patient on optimum anticholinesterase treatment is less than 2 litres. Additional features associated with a probable need for artificial ventilation were the presence of a thymoma, bulbar symptoms, especially dysphagia, and age over 50 years. These should be taken into account in any patient whose vital capacity is close to the critical level of 2 litres. When postoperative ventilation was required it was usually necessary for 12 days or more, and tracheostomy should therefore be done at or before thymectomy. Most patients in this series received the same dose of anticholinesterases after operation as before it and no evidence was found of a sudden decrease in requirements for anticholinesterase therapy. Two patients did not, and in them a myasthenic crisis was precipitated. We propose that the preoperative drug regimen can be continued in the immediate postthymectomy period, allowing selection of patients for tracheostomy and artificial ventilation primarily on the basis of the preoperative vital capacity.