We assessed the relation between smoking and pneumothorax in patients aged <40. Of 526 patients who underwent surgery for pneumothorax in 2011~2015, 311 were under the age of 40 and they were included in this study. Of 311, 54 cases( 17.4%) were diagnosed with spontaneous pneumothorax associated with emphysematous change by pathological assessment. By the multivariate analysis, 2 parameters exhibited significance for the risk of these spontaneous pneumothorax:a Brinkmann index of ≥165 and a smoking period of ≥9.5 years. It was suggested that smoking is strongly related to the onset of spontaneous pneumothorax even in the younger population less than 40 years of age.