Vitamin D and its metabolites, inhibit the growth of mycobacteria in macrophages and shift the immune response from Th1 to Th2. In our study, the role of 25 (OH) vitamin D in the pathogenesis of tuberculosis was investigated by determining its serum levels in active pulmonary tuberculosis. The serum 25 (OH) vitamin D levels were 21.06±16.51 ng/ml (Mean±SD) in active pulmonary tuberculosis patients (n=44), 81.95±67.87 ng/ml in inactive pulmonary tuberculosis patients (n=24) and 67.0±53.25 ng/ml in healthy control subjects (n=20). 25 (OH) vitamin D levels were significantly different in active pulmonary tuberculosis patients when compared with inactive pulmonary tuberculosis and healthy control subjects (p<0.001, for both comparisons). Serum 25 (OH) vitamin D levels increased after treatment in 10 patients in which 25 (OH) vitamin D levels were determined before and after treatment (p<0.05).In conclusion, the decreased 25 (OH) vitamin D concentrations in patients with active tuberculosis may facilitate the development of active infection.