The aim of this study was to determine whether control status of diabetes mellitus influences clinical and radiographic manifestations and treatment responses in patients with tuberculosis (TB). The medical records of 492 patients who started anti-TB medication between January 2005 and December 2009 were retrospectively reviewed. Diabetes was diagnosed in 124 patients (25.2%). Of these, 74 (59.7%) were uncontrolled (HbA1C≥7.0), 25 (20.2%) were controlled (HbA1C<7.0), and HbA1C levels were not assessed in the remaining 25 (20.2%). There were no differences in clinical symptoms between diabetics and non-diabetics, regardless of diabetes control status. There were also no differences in radiographic findings or AFB results between controlled diabetics and non-diabetics. However, uncontrolled diabetics had more cavitary lesions (p=0.008) and higher positive smear rates (p<0.001) compared with non-diabetics. After adjustment for age, cavities and positive smears before initiation of treatment, uncontrolled diabetes was a significant risk factor for a positive sputum culture at 2 months (odds ratio, 4.316; 95% CI, 1.306-14.267; p=0.017). Uncontrolled diabetics seem to have more cavities, higher positive smear rates and lack of culture conversion after two months of therapy. Therefore, TB patients with uncontrolled diabetes should be carefully managed and treated.