Aim. To investigate the clinical and functional parameters in patients with asthma-chronic obstructive pulmonary disease overlap syndrome (ACOS) versus those with chronic obstructive pulmonary disease (COPD) and asthma. Subjects and methods. A total of 129 people were examined. 51 patients with ACOS were followed up in Group 1; Group 2 included 38 patients with severe asthma; Group 3 consisted of 40 patients with severe COPD. All the patients underwent clinical examination: history data collection, physical examination, evaluation of disease symptoms, and study of respiratory function (spirometry, body plethysmography). Results. ACOS is clinically characterized by considerable demands for emergency drugs and by more frequent asthmatic fits and exacerbations, which require hospitalization. The parameters of bronchial resistance in ACOS were established to be increased throughout the follow-up period and to be comparable with those in patients with COPD. In the patients with ACOS, the severity of pulmonary hyperinflation was associated with increased demands for emergency drugs (r=0.59; p=0.015). Fixed bronchial obstruction in ACOS can be caused by smoking intensity and duration associated with increased bronchial resistance in expiration (r=0.51; p=0.003) and intrathoracic volume (r=0.71; p=0.0001); as well as increased body mass index (p<0.001) and disease duration, which were interrelated with a reduction in the forced expiratory volume in one second/forced vital capacity ratio (r=–0.63; p=0.001 and r=–0.71; p=0.0034, respectively). Conclusion. Patients with ACOS show more severe clinical manifestations and a substantial increase in functional residual capacity and intrathoracic volume throughout the follow-up period, suggesting that the distal bronchi are impaired and pulmonary hyperinflation develops.