Background: A mismatch between a smaller donor lung allograft and a larger recipient thorax has been associated with inferi-or posttransplant survival. However, some controversy exists regarding the association of undersized lung allograft with poor clinical outcomes of lung transplantation (LTx), and the available data are still limited. This study evaluated long-term outcomes, including pulmonary function of undersized allograft compared to well-matched allograft. Methods: Between March 2015 and June 2020, a total of 188 patients received LTx were registered in the prospective, multi-center KOTRY registry. The donor-to-recipient size matching was classified using the donor-to-recipient predicted total lung ca-pacity (pTLC) ratio. After excluding patients received oversized allografts (pTLC ratio of >1.1, n=82), patients were divided into an undersized group (allografts with pTLC ratio of <0.9, n=23) and a well-matched group (allografts with pTLC ratio of between 0.9 and 1.1, n=83). Results: During the immediate posttransplant period, there was no difference in primary graft dysfunction, operation-related complications, hospital mortality, and functional status at discharge. Also, there was no difference in mortality, acute cellular rejection, and chronic lung allograft dysfunction during the long-term follow-up period. However, in the serial follow-up of pul-monary function, FEV1 was significantly decreased in the undersized group than the well-matched group at 3 months (49.0% vs. 66.0%, P=0.038), 6 months (60.5% vs. 75.5%, P=0.023), 1 year (57.0% vs. 81.0%, P=0.046) and 2 years (63.0% vs. 77.0%, P=0.055). FVC was also significantly decreased in undersized group than well-matched group at 3 months (44.0% vs. 61.0%, P=0.033), 6 months (53.0% vs. 64.5%, P=0.037), 1 year (59.5% vs. 75.5%, P=0.039), and 2 years (57.0% vs. 75.0%, P=0.029). Conclusions: Undersized allografts based on pTLC ratio is associated with poor long-term pulmonary function than well-matched allografts, although there are no differences in clinical outcomes.