There are several limitations to this study. First, our study was retrospective in design. However, the subjects were from prospectively recruited cohorts with little missing data. Another limitation was that the pathologic pattern of 30.5% of the patients with UCTD-IP was not confirmed. However, the only significant difference between UCTD-UIP and UCTD-NSIP was age (Table 2) and the age of UCTD-undetermined pathology group was similar to the UCTD-UIP group so as to the survival curves. If all of these patients had had an NSIP pattern, the results would not have been different except that the prevalence of UCTD-NSIP would have increased to 49.5%. Because the old age (> 65 years, particularly among males) is strongly associated with UIP-pattern pathology in patient IIP but without the typical UIP pattern of HRCT scans (without honeycombing),25 we reanalyzed the data as old age male patients with UCTD without biopsy (n = 2) having UCTD-UIP. The results were the same (better survival of UCTD-UIP, hazard ratio, 0.529; 95% CI, 0.302-0.928; P = .026). Third, although we adopted narrow criteria of UCTD including a high titer of ANA (> 1:160), it may be somewhat liberal and ANA titer could change, if repeated. In the subjects, ANA titer was 1:1280 in six patients, 1:640 in nine, 1:320 in 23, and 1:160 in 16 patients. Repeated tests were performed in 28 patients; the results were the same (> 1:160) in 25 patients. The titer was reduced in three patients, however, they were measured during the therapy with steroid and immunosuppressant. Furthermore, there is an innate difficulty in distinguishing arthralgias from osteoarthritis. However, most of the subjects, especially with UCTD were seen by a rheumatologist, and we think the possibility of misclassification was low. Finally, although this is the largest series of UCTD-IP cases reported to date, the numbers are still relatively small and from a single center. Thus, a multinational multiinstitutional prospective study with a larger number of patients is needed in the future.