We have previously shown that the chemosensitivity of the respiratory centers is well preserved in myotonic dystrophy but that the ventilatory output is reduced. The present study was designed to determine at which degree of ventilatory performance weakness and fatigability of the respiratory muscles are interfering with ventilation and which mechanical factors contribute to the tachypnea of patients with myotonic dystrophy at rest and during low ventilatory output. We studied 10 patients with the disease and 10 normal control subjects. The strength of respiratory muscles was assessed by measurements of maximal pressure-volume diagrams generated against airway occlusion. Performance was evaluated during 1-min maximal voluntary ventilation (1-min MVV) test, during 7-min 7% CO2 breathing and during quiet breathing. Occlusion pressure (P0.1) in patients at rest was slightly higher than in control subjects, and during CO2 breathing, it was similar to that of control subjects. Maximal static pressure was reduced in patients to an average of 35% of that of control subjects. During the 1-min MVV test, there was a 50% reduction in esophageal and transdiaphragmatic pressure output (Pes, Pdi) in patients, resulting in similar reduction in ventilation (VE) and patients had rapid cycles of alternating dominant thoracic and abdominal volume displacements (Vrc/Vabd) suggesting respiratory muscle fatigue. During the 3- to 4-fold increase in breathing drive induced by hypercapnia, pressure output and the Vrc/Vabd were identical in both groups. However, ventilation was reduced in patients who had tachypneic respiration. In patients, tachypnea was also observed during quiet breathing. This tachypnea was associated with higher impedance of the respiratory system (Zrs) in patients and identical impedance of the lung (ZL) in both groups. In addition, Pdi during tidal volume was significantly higher in patients. These data demonstrate that the ventilatory output in out patients was altered predominantly by weakness and fatigability of the respiratory muscles during high ventilatory performance and by increased impedance of the respiratory system at lower degrees of ventilation.