OBJECTIVE: To evaluate the difference in forced vital capacity (FVC) between sitting and supine position in patients with amyotrophic lateral sclerosis (ALS), cervical spinal cord injury (SCI) and Duchenne muscular dystrophy (DMD). METHOD: FVC was measured in sitting and supine position for 32 patients with DMD, 32 patients with cervical SCI and for 28 patients with ALS. The highest value in three or more attempts in each position was chosen. RESULTS: FVCs measured in cervical SCI and ALS patients in the sitting and supine position were 1612.8+/-291.0 ml, 1393.2+/-286.7 ml and 2054.7+/-545.8 ml, 1104.3+/-425.4 ml respectively. Cervical SCI patients showed significantly higher value in the supine position (p<0.05). And ALS patients showed significantly higher value in the sitting position (p<0.05). FVCs measured in DMD patients were 1311.6+/-260.7 ml and 1213.8+/-378.9 ml respectively. There was no statistically significant difference between the measurements in both positions. CONCLUSION: Difference in postural change of FVC was observed in patients with different types of neuromuscular disorders. Such difference in FVC suggest that postural change of FVC should be considered in management of neuromuscular disease with respiratory muscle weakness.