The objective of the study was determination of the most frequent bacterial factors, including Haemophilus parainfluenzae, suspected of causing COPD exacerbation, of the relation between bacterial strains and respiratory system functional status as well as of antibiotic sensitivity of sputum isolated bacteria. The examined group comprised 28 patients treated in the Pulmonary Department of Medical University of Lublin. The subjects fulfilled the criteria of type I COPD bacterial exacerbation. Patient's chest x-ray and spirometry tests were performed. Forty-nine bacterial strains were isolated. In the case of nine patients, more than one strain was isolated. Subjects having H. parainfluenzae in sputum had significantly higher (p<0.05) FVC and FEV1 values comparing to patients with H. influenzae or other Gram-negative bacteria. H. parainfluenzae may be an important etiologic factor of COPD exacerbation. Aetiology of bacterial COPD exacerbation depends on the level of respiratory parameter limitation.