question: BACKGROUND: The aim of this study was to investigate the factors affecting pneumothorax development, lung expansion difficulty, and the impact on mortality in novel coronavirus 2019 (COVID-19) patients. METHODS: Between March 2020 and January 2021, a total of 6,108 COVID-19 patients (3,050 males, 3,058 females; mean age: 49±17.6 years, range, 12 to 89 years) who presented to our hospital were retrospectively analyzed. The patients were divided into two groups. Group 1 (the COVID-19-pneumothorax group) consisted of patients developing pneumothorax while under treatment for COVID-19. Group 2 (COVID-19-without pneumothorax group) consisted of consecutive patients with pulmonary involvement during the same period, requiring intensive care, but not developing pneumothorax. Biochemical test results were examined for the day of hospitalization, day of transfer to intensive care unit, and Day 5 of intensive care unit stay. Lung parenchyma involvement rates on thoracic computed tomography, duration of drainage, duration of intensive care unit stay, and morbidity/mortality data were investigated. RESULTS: Of all patients with COVID-19, 2,342 were admitted to our clinic and 460 needed intensive care. Pneumothorax developed in 21 patients (0.34% of all patients and 0.9% of hospitalized patients). Significant involvement diffusion on computed tomography, ferritin and D-dimer elevation, and low lymphocyte (%) values were observed in the COVID-19 patients with pneumothorax. Pneumothorax development, and ferritin and D-dimer elevation were most correlated with mortality at regression analysis. There was a significant difference in difficulty in lung expansion between COVID-19 patients who developed pneumothorax and who did not. CONCLUSION: Pneumothorax may develop in COVID-19 patients whose biochemical parameters worsen, and expansion difficulty may be experienced, which is likely to be associated with mortality.