Backgrounds: The aim of the study is to estimate the unrecognized in-hospital days of TB patients without isolation, using national health insurance claim data. Methods: Total patients who had record of drug susceptibility test for TB during 2012~2016, which means culture-proven tuberculosis were included. Among them, patients with admission records during infectious periods were selected, and their in-hospital days were calculated after subtracting periods of isolation. Infectious periods were defined according to the CDC’s guideline for contact investigation. The first day of infectious periods was simply defined as 3 months before doctor’s suspicion of TB in patients who received the prescription for respiratory medications, and 1 month before without prescription records of respiratory medications. The last day of infectious periods was defined as a day before the first prescription of anti TB medications. Results: From 2012 to 2016, 7186 cases were hospitalized for at least one day without isolation. Total sum of in-hospital days without isolation were 94636 person-days during 5 years - 13.17 days per one case. Patient over the age of 60 accounts for 63.9% of total cases and 69.7% of total in-hospital days. Hospitalized patients showed longer hospital delay compared to total patients (median 23 days versus 2 days). Hospitalized patients had more comorbidities than those who were not hospitalized as calculated with Charlson comorbidity index (mean 2.98 versus 2.04, p＜0.0001). Conclusions: The burden of nosocomial transmission of TB is still substantial in Korea.