Background: The aim of our study was to evaluate the "diagnosis change" rate among patients notified as tuberculosis (TB) to Korean TB surveillance system (KTBS). Methods: 1,273 patients notified to KTBS in one private university hospital in 2011 were enrolled. Patients were classified into three groups, "Active TB" (bacteriologically-confirmed or clinically diagnosed), "Diagnosis change" (initially notified as TB, but finally diagnosed as non-TB) and "Administrative error" (notified as TB due to administrative errors). Results: Except 17 patients (0.13%) in "Administrative error", "Active TB" and "Diagnosis change" included 1,097 (86.2%) and 159 patients (12.5%), respectively. There was no significant difference in the clinical characteristics between two groups. Common causes of "diagnosis change" were nontuberculous mycobacterial (NTM) disease (38.4%, 61/159), pneumonia (13.2%) and lung cancer (10.1%) in cases notified as pulmonary TB, and meningitis (5.0%) and Crohn`s disease (3.1%) in cases notified as extrapulmonary TB. Old age (≥55 years, OR=2.75), high body mass index (≥30 kg/m2, OR=3.46), and absence of TB-sugges-tive radiologic findings (OR=2.22) were predictors for "diagnosis change" in a multivariate analysis. Conclusions: Because of high rate of "Diagnosis change" in a TB notification to KTBS, TB incidence rate by KTBS maybe overestimated. Thus, when reporting annual TB notification rate in Korea, the number of patients with "Diagnosis change" should be excluded. The most common cause of "Diagnosis change" was due to NTM disease.