Factors that relate to medium-term outcome in patients with pulmonary tuberculosis (PTB) who have completed the 2-month intensive phase of treatment are incompletely understood. The relationship between in vitro production of interferon-gamma (IFN-gamma), interleukins (ILs)-5 and -10 and drug levels determined after 2 months of drug therapy, to outcome at 24 months was studied prospectively. Cytokine concentrations were determined from culture supernatants after stimulation of whole blood with purified protein derivative (PPD) of Mycobacterium tuberculosis. Plasma concentrations of rifampin, isoniazid, pyrazinamide and ethambutol were determined by high-performance liquid chromatography. The treatment failure and relapse free survival probability was 0.54 (95% CI: 0.40-0.67) at 24 months. In multivariate analysis of parameters at 2 months the strongest positive associations with disease free survival were IFN-gamma response to PPD (p=0.002) and serum creatinine (p=0.001). Drug concentrations were not associated with outcome although rifampin exposure correlated with IFN-gamma response to PPD (p=0.0132). These data suggest that the ability to mount a recall immune response to M. tuberculosis may influence treatment outcome. The data support the idea to identify persons at risk of a poor treatment outcome by monitoring of the in vitro response to tuberculosis antigens.