A prospective study of 50 consecutive patients presenting with tuberculosis has shown that patients have on average lower serum concentrations of 25-hydroxycholecalciferol (25-OHD) than healthy matched controls. No difference was shown in serum 1,25-(OH)2D, 24,25-(OH)2D, or parathyroid hormone. Uncorrected serum calcium was lower in the patient group but identical when correction concentrations were made for albumin. Serum liver enzyme concentrations were significantly higher in the patient population. Low serum vitamin D concentrations may be a consequence of disease. The possibility that low serum 25-OHD concentrations may predispose to tuberculosis infection cannot, however, be excluded. Prolonged treatment with isoniazid or rifampicin, both of which have been shown to reduce serum 25-OHD, may increase the risk of vitamin D deficiency and consequent osteomalacia in groups of patients most at risk, such as those of Indian subcontinental ethnic origin.