In myasthenia gravis (MG) the status of respiratory function has a paramount importance and a careful evaluation is recommended. The weakness of respiratory muscles has been demonstrated in several studies. However, a reliable simple method for the evaluation of this muscular group was lacking until recently, when the usefulness of the maximum respiratory pressures, expiratory (MEP) and inspiratory (MIP), was demonstrated. We evaluated with this method a series of 23 patients with a diagnosis of MG (16 females and 7 males), with a mean age of 46 years (22-68 years), clinically stable and without symptomatic dyspnea. They were distributed in: grade I (5), grade II A (12), and grade II B (6). All of them were evaluated with flow-volume curves, pletysmography, gas transfer, MEP and MIP. The resulting values were then correlated with the expected ones, a reduction greater than one SD being considered as abnormal. The results showed that respiratory function was normal without a restrictive pattern. However, the force of respiratory muscles was reduced in the following proportions of patients in the different groups: grade I: MIP 40%, MEP 60%; in grades II A and II B both MEP and MIP were reduced in 84% of patients. When a statistical comparison with the expected values was carried out it was found that MEP and MIP, considered as a group, were reduced to 53% of the expected values (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)