[Purpose] Mechanical insufflation-exsufflation (MI-E) significantly improved atelectasis of the right medial lobe, lower lobe and lower left lobe, and thoracic compliance (Cdyn) in a subject with severe motor and intellectual disabilities (SMID). [Participant and Methods] The subject was a 30-year-old with SMID. Ventilator management under tracheotomy for quadriplegia, scoliosis with Cobb angle 92.8°, and passive right lateral decubitus posture was incomplete. Postural drainage + MI-E was performed for atelectasis of the right middle lobe and the lower lobe. Due to the severity of scoliosis, insufficient postural drainage + MI-E was performed on the lower left lobe. [Results] Atelectasis of the middle and lower lobes of the right lung disappeared, but partial atelectasis of the left lower lobe did not improve. Cdyn improved significantly after 6 weeks of intervention. [Conclusion] Insufficient postural drainage did not improve atelectasis. It took one month to improve Cdyn.