Macrophage migration inhibitory factor (MIF) was originally described as a T-cell-derived cytokine that inhibits the random migration of macrophages and promotes the delayed-type hypersensitivity reaction. MIF plays an important role in the regulation of the Th1/Th2 balance in inflammatory response. This study investigated serum levels of circulating MIF in patients with pulmonary tuberculosis. The levels of MIF in sera were measured by enzyme-linked immunosorbent assay in 34 patients with pulmonary tuberculosis (16 males and 18 females) and 30 healthy controls (15 males and 15 females). The mean levels of circulating MIF values were significantly higher in those with pulmonary tuberculosis (19.84 +/- 11.27 ng/ml; P < 0.0001) than in the healthy controls (4.38 +/- 1.34 ng/ml). Circulating MIF values significantly correlated with circulating interferon-gamma values (r = 0.537, P < 0.0001). Thus, MIF may play an important role in immune responses to human infection with Mycobacterium tuberculosis.