['Vitamin D deficiency is associated with susceptibility to active tuberculosis (TB) in many settings. In vitro studies and studies on human volunteers showed that two of the first-line anti-tuberculosis drugs, isoniazid and rifampicin, reduce 25-hydroxy vitamin D (25[OH]D) concentrations.', 'To study changes in vitamin D status during treatment of Tanzanian hospitalised patients with pulmonary TB (PTB).', 'We compared serum 25[OH]D concentrations in 81 Tanzanian PTB patients before and after 2 months of treatment.', 'Median serum 25[OH]D concentrations increased from 91 nmol/l at baseline to 101 nmol/l after 2 months of TB treatment (median increase 6.0 nmol/l, IQR -0.7-25.0, P = 0.001). Median serum parathyroid hormone concentrations increased from 1.6 to 2.0 pmol/l (median increase 0.46, IQR -0.2-1.1, P < 0.001).'] Do serum 25-hydroxy-vitamin D3 concentrations increase during tuberculosis treatment in Tanzania?