Vitamin D, a modulator of macrophage function, can activate human anti-mycobacterial activity. Vitamin D deficiency is therefore associated with a higher risk of tuberculosis (TB) infection, as indicated by several observations. First, TB tends to occur during the colder seasons when cutaneous synthesis of vitamin D from sun exposure is reduced and serum vitamin D levels are lower. Second, patients with untreated TB, particularly those from a temperate climate, have lower serum vitamin D levels than healthy subjects. Third, the incidence of TB is higher among subjects with relatively low serum vitamin D levels, such as the elderly, uremic patients, and Asian immigrants in the U.K.