Pneumothorax associated with thoracic endometriosis (TE) generally occurs in women around 30 years old and it usually affects the right pleural cavity. We herein report two cases of TE associated with left-sided pneumothorax in young women. The prevalence of TE in younger patients may be underestimated if these cases are treated as spontaneous pneumothorax. Pneumothorax occurring in younger patients has not been reported to show laterality. TE-related or catamenial pneumothorax in young women must therefore represent a different clinical entity from the condition seen in older patients.