The relationship between tumor necrosis factor (TNF)-alpha level, body temperature, and pulmonary lesion score was determined in 3-week-old pigs inoculated intranasally with swine influenza virus (SIV) subtype H1N2. The expression of TNF-alpha was measured in bronchoalveolar lavage (BAL) fluids by enzyme-linked immunosorbent assay and lung tissues by immunohistochemistry. In BAL fluid, TNF-alpha concentration was maximal at 1 days post-inoculation (dpi), declined markedly by 3 dpi (P<0.05) and steady thereafter. Mean rectal temperatures were above 40 degrees C for the infected groups at 1 dpi and declined markedly by 3 dpi. The body temperatures were correlated with the levels of TNF-alpha in BAL fluids from pigs experimentally infected with SIV (r(s)=0.9754, P<0.05). The pulmonary lesion scores were correlated with the means of positive cells by immunohistochemistry for TNF- alpha (r(s)=0.9949, P<0.001). The results suggest that the expression of TNF-alpha protein plays an important role in induction of pulmonary lesion and clinical sign such as fever in SIV infection.