In order to determine predictors for the postoperative need of mechanical ventilation in patients with myasthenia gravis undergoing thymectomy, the authors retrospectively applied multivariate discriminant analysis to preoperative physical, historical, and laboratory data of 24 myasthenic patients. They identified four risk factors--duration of myasthenia, respiratory disease, pyridostigmine dosage, and vital capacity--that allowed prediction of which patients would need postoperative mechanical ventilation and which could readily have their tracheas extubated. The four factors were weighted according to their respective importance in making this prediction and combined to form a preoperative scoring system. Using the resultant scores for each patient, the authors correctly predicted ventilatory need in 91 per cent of the patients, and only conservative errors (predicting the need for ventilatory support) were made. Traditionally used criteria for evaluating myasthenic patients were poorer predictors than the four factors identified by the authors. The scoring system may be clinically useful in the preoperative evaluation and postoperative care of the patient with myasthenia gravis, for its identifies important variables in the evaluation of the myasthenic patient and serves as an aid to the physician in identifying those able to tolerate early tracheal extubation.