Twenty consecutive patients (16 women and 4 men), with a mean age of 40 years, who were diagnosed and treated for myasthenia gravis were enrolled in a prospective investigation aimed at determining the amount of respiratory disturbance occurring during sleep while they received treatment. Patients were clinically evaluated to determine body mass index, presence of upper airway anatomical abnormalities, level of functional capacity and activity scored from 1 to 5, and presence of sleep-related complaints. They underwent daytime pulmonary function tests, determination of maximal static inspiratory pressure, measurement of transdiaphragmatic pressure, and measurement of arterial blood gas levels. Polygraphic monitoring during sleep, evaluating respiration and oxygen saturation, was also performed. Results indicated that in the studied population, all subjects had evidence of daytime diaphragmatic weakness as demonstrated by transdiaphragmatic pressure measurements, independent of the degree of autonomy and functional capacity and activity level reached. Older patients with moderately increased body mass index, abnormal total lung capacity, and abnormal daytime blood gas concentrations were the primary candidates for development of diaphragmatic sleep apneas and hypopneas, and oxygen desaturation of less than 90% during sleep. However, these clear indicators were not found in all subjects with sleep-related disordered breathing. Rapid-eye-movement sleep was the time of highest breathing vulnerability during sleep. Sleep-related complaints may also help identify subjects at risk for abnormal breathing during sleep, even when daytime functional activity is judged normal.