Background Diaphragm motion during forced expiration can be analyzed using M-mode ultrasound in an anterior subcostal approach. Maximal expiratory diaphragmatic excursion and forced expiratory diaphragmatic excursion in the first second physiologically mimic forced vital capacity (FVC) and forced expiratory volume in the first second (FEV1), respectively, and may be used as a marker of obstruction. Aim The aim of the work was to assess the role of the M-mode index of obstruction (MIO) as a screening test (tool) for chronic obstructive pulmonary disease (COPD). Patients and methods This case–control study involved 200 participants (100 patients with COPD and 100 age-matched and sex-matched healthy controls). The authors performed spirometry and the diaphragm ultrasonography, during forced expiration. MIO was calculated as the slope of diaphragmatic excursion in first second/slope of diaphragmatic excursion at end of expiration. Results Diaphragmatic excursion was significantly lower in COPD group than control group (4.27±1.49 vs 5.36±1.67 for slope in the first second of expiration, 4.82±1.55 vs 5.72±1.57 for maximum slope at the end of expiration, 4.42±1.53 vs 5.44±1.69 for velocity of diaphragm contraction in first second, and 2.40±1.04 vs 3.52±1.26 for velocity of diaphragm contraction at the end of expiration; P= Conclusion Diaphragmatic excursion during forced expiration is significantly decreased in COPD in comparison with the control group. MIO is significantly lower in COPD in comparison with control and significantly correlated with FEV1/FVC.