AbstractSmall-bore thoracic catheter drainage is recommended for a first large or symptomatic episode of primary spontaneous pneumothorax (PSP). However, one-third of these patients require a second procedure because of treatment failure. We investigated the factors associated with unsuccessful pigtail catheter drainage in the management of PSP. In this retrospective study, using a prospectively collected database, we enrolled 253 consecutive patients with PSP who underwent pigtail catheter drainage as initial treatment, from December 2006 to June 2011. The chest radiograph was reviewed in each case and pneumothorax size was estimated according to Light’s index. Other demographic factors and laboratory data were collected via chart review. Pigtail catheter drainage was successful in 71.9% (182/253) of cases. Treatment failure rates were 42.9%, 25.9%, and 15.5% in patients with pneumothorax sizes of >62.6%, 38–62.6%, and <38%, respectively (tertiles). An alternative cut-off point of 92.5% lung collapse was defined using a classification and regression tree method. According to the multivariate analysis, a large-size pneumothorax (p = 0.009) was the only significant predictor of initial pigtail catheter drainage treatment failure in patients with PSP. Early surgical treatment could be considered for those patients with a large-sized pneumothorax.