Background: COPD patients have reduced diaphragmatic mobility and thoracoabdominal asynchrony (TAA), reducing ventilatory efficiency. Aim: To investigate the association between TAA and diaphragmatic mobility (DM) in COPD patients. Methods: Non-obese men with moderate to very severe COPD were included. DM (B-mode ultrasonography) and TAA (optoelectronic plethysmography, OEP) were assessed one week apart. DM was assessed in four conditions: regular and forced breathing and pre and post-60-sec sit-to-stand test. The mean value of three measurements (<5%variation) was used for analysis. OEP assessed the three compartments: upper (URC) and lower (LRC) rib cages and abdomen (ABD), and total chest wall volume. OEP assessment was performed at rest and during exercise on a stationary bicycle (25%Wpeak). The phase shift method quantified TAA between compartments (URC, LRC, and ABD). A blinded assessor assessed movement in opposite directions using Matlab routine software. Degrees ranged from 0% (perfect synchrony) to 180% (total asynchrony). Results: Patients were 66±8 years, eutrophic (body mass index, 22±3.7kg/m²), and had severe to very severe airway obstruction (39±11%pred FEV₁). Change in post-exercise test regular breathing DM was linearly correlated with exercise TAA between URC vs. ULC (r=-78;p=0.04) and LRC vs. ABD (r=0.82;p=0.03). Exercise TAA URC vs. ULC was correlated with pre-test forced breathing DM (r=-785; p=0.04). Conclusion: Diaphragm mobility assessed by ultrasound is strongly associated with thoracoabdominal asynchrony in patients with moderate to very severe COPD.