Background: Low serum concentrations of drugs for multi-drug resistant tuberculosis (MDR-TB) have occasionally been associated with treatment failure. OBJECTIVE: We determined the prevalences of low serum concentrations of anti-MDR-TB drugs and the effects of serum levels according to the presence of 2 month-sputum conversion. Design: Venous blood was obtained 2hrs~6hrs after drug ingestion among 29 MDR-TB patients during each monthly visit, and the serum levels of moxifloxacin (Mf), prothionamide (PTH), and cycloserine (CS) were measured using high pressure liquid chromatography/tandem mass spectrometry (LC-MS/MS) method. Results: Among 89 blood samples, the prevalences of low serum concentrations of Mf, PTH, and CS below the minimal level of therapeutic range were 70.8% (17/24), 60.0% (27/45), and 75.0% (39/52), respectively. There were no differences between 2 month-sputum conversion group (n=25) and 2 month-sputum non-conversion group (n=4) in median drug concentration (μg/ml) of Mf (1.46 VS. 1.60), PTH (0.91 VS. 0.70), and CS (14.90 VS. 14.90), respectively (p＞0.05). However, poor compliance rate was significantly higher in 2 month-sputum non-conversion group (75.0%, 3/4) than in 2 month-sputum conversion group (0%, 0/25) (p＜0.01). Conclusion: The prevalences of low concentrations of anti-MDR-TB drugs were substantial. Prospective larger studies with timely samplings are needed to investigate the role of therapeutic drug monitoring.