Multicore myopathy is a rare congenital myopathy that can cause progressive weakness, but it has not been recognized previously to have respiratory consequences. This study describes two patients who developed respiratory failure because of respiratory muscle weakness. Both patients had low vital capacities without evidence of airway obstruction, and CO₂ retention. Physical examination found that neck accessory muscles and abdominal muscles were very weak. In inspiration the abdomen expanded, but the rib cage contracted. Detailed studies were done in one patient with magnetometers, esophageal and gastric pressures, flow and volume, and electromyograms of abdominal and neck accessory muscles. Peak static inspiratory and expiratory pressures were 28 and 30 cm H₂O, respectively, and maximal transdiaphragmatic pressure was 28 cm H₂O. Vital capacity was reduced to 44% of predicted. Lung compliance was normal. In spontaneous breathing while supine, rib-cage diameters decreased while gastric pressure increased in inspiration, suggesting the weak diaphragm was the main muscle of respiration. EMG recordings showed no evidence of recruitment of the wasted neck accessory muscles (sternocleidomastoid and scalene), whereas electromyograph plus mechanics measurements gave evidence of abdominal muscle use in the sitting but not the supine posture. More limited studies in the second patient gave similar results. Multicore disease in these two patients thus caused marked weakness of all respiratory muscles, affected the intercostal-accessory group more than the diaphragm, and led to respiratory failure.