Introduction: Undifferentiated connective tissue disease (UCTD) defined as conditions characterised by the presence of signs and symptoms suggestive of connective tissue disease (CTD) with autoantibody but do not fulfill classification criteria of specific CTD. Although NSIP pattern was first described in UCTD, UIP pattern seems to be not infrequent, and then the differentiation from idiopathic pulmonary fibrosis may be difficult. The aim of this study was to investigate the frequency and prognosis of UIP and NSIP pattern in UCTD and prognostic difference between UCTD-UIP and IPF. Methods: The medical records including rheumatologic symptoms and serologic test for autoantibody of 788 patients diagnosed as IIP from January 2005 to December 2012 at AMC were retrospectively reviewed. UCTD was diagnosed according to narrow criteria and the prognosis was compared between UCTD-UIP and UCTD-NSIP and also between UCTD-UIP and IPF. Results: Total 105 patients (13.3%) diagnosed as UCTD and 44 patients had UIP pattern and 29 patients had NSIP pattern, 8 OP patterns and the pathologic pattern was not specified in remaining 24 patients. Overall survival of UCTD-UIP group was shorter than UCTD-NSIP group (p=0.021) but significantly better than that of IPF group(p=0.042). Definite CTD developed in 2.9% (3/105) of patients during follow-up days. Conclusion: UIP pattern is frequent in UCTD related interstitial pneumonia and the prognosis was worse than UCTD-NSIP. However, the prognosis of UCTD-UIP was better than IPF, it is important to search for UCTD in suspected IPF.Introduction: Undifferentiated connective tissue disease (UCTD) defined as conditions characterised by the presence of signs and symptoms suggestive of connective tissue disease (CTD) with autoantibody but do not fulfill classification criteria of specific CTD. Although NSIP pattern was first described in UCTD, UIP pattern seems to be not infrequent, and then the differentiation from idiopathic pulmonary fibrosis may be difficult. The aim of this study was to investigate the frequency and prognosis of UIP and NSIP pattern in UCTD and prognostic difference between UCTD-UIP and IPF. Methods: The medical records including rheumatologic symptoms and serologic test for autoantibody of 788 patients diagnosed as IIP from January 2005 to December 2012 at AMC were retrospectively reviewed. UCTD was diagnosed according to narrow criteria and the prognosis was compared between UCTD-UIP and UCTD-NSIP and also between UCTD-UIP and IPF. Results: Total 105 patients (13.3%) diagnosed as UCTD and 44 patients had UIP pattern and 29 patients had NSIP pattern, 8 OP patterns and the pathologic pattern was not specified in remaining 24 patients. Overall survival of UCTD-UIP group was shorter than UCTD-NSIP group (p=0.021) but significantly better than that of IPF group(p=0.042). Definite CTD developed in 2.9% (3/105) of patients during follow-up days. Conclusion: UIP pattern is frequent in UCTD related interstitial pneumonia and the prognosis was worse than UCTD-NSIP. However, the prognosis of UCTD-UIP was better than IPF, it is important to search for UCTD in suspected IPF.