OBJECTIVE: To evaluate the difference in vital capacity (VC) between sitting and supine position in patients with amyotrophic lateral sclerosis (ALS) and Duchenne muscular dystrophy (DMD). METHOD: VC was measured in the sitting and supine position for 30 patients with DMD and for 30 patients with ALS. The highest value in three or more attempts in each position was chosen. RESULTS: VCs measured in ALS patients in the sitting and supine position were 1591.7+/-634.6 ml and 1290.0+/-580.3 ml respectively. The VC in the sitting position showed significantly higher value than the VC in the supine position (p<0.05). VCs measured in DMD patients were 903.7+/-518.1 ml and 795.3+/-505.6 ml respectively. There was no statistically significant difference between the measurements in both positions. CONCLUSION: Difference in postural change of VC was observed in patients with different types of neuromuscular disorders. Such difference in VC suggest that postural change of VC should be considered in management of neuromuscular disease with respiratory muscle weakness.