For non-ambulatory patients with flaccid type of neuromuscular scoliosis (fNMS), surgery offers various potential benefits. However, increased sitting height is a significant issue that may lead to impaired ADL in patients with upper limb muscle weakness. In this study, we elucidated sitting height changes after fNMS surgery, and characteristics of patients with highly increased sitting height. 80 fNMS patients (55 males, 25 females; mean age :13.7 years) who underwent NMS surgery were included. We measured T1-S height and length in the preoperative sitting and supine positions, as well as in the sitting position at 1 month and 1 year postoperatively. We also measured the Cobb angle, pelvic obliquity (PO), and spinal pelvic obliquity (SPO). Then we compared two groups based on the presence or absence of 10 cm or greater increase in T1-S height or length at 1 month postoperatively. The T1-S height/length in the sitting position after fNMS surgery showed an average increase of 7.2/7.0 cm, representing an average increase of 1.5/1.4 cm compared to the preoperative supine position. The group with T1-S height or length increase over 10 cm comprised 16 cases (20%). Their preoperative, 1-month postoperative, and 1-year postoperative Cobb angles were 98°/49°/49°, PO was 35°/17°/20°, and SPO of 43°/23°/29°. Compared to the other group's Cobb angles of 82°/35°/39°, PO of 30°/13°/16°, and SPO of 38°/16°/23°, the Cobb angles at preoperative and 1-month postoperative periods and the SPO at 1-month postoperative period were significantly higher. Following fNMS surgery, sitting height increased by an average of over 7 cm, with an additional increase of about 1.5 cm compared to the supine position. About 20% of cases experienced a sudden change in posture resulting in an increase of 10 cm or more postoperatively, and it was found that these cases predominantly involved patients with severe preoperative spinal deformity which might be helpful information for informed consent and surgical strategy. By contrast, we experienced cases where patients became unable to eat independently after surgery, thus, further investigation of clinical symptoms is considered necessary.