The aim was to find out how the IL-12 and IL-18 levels in the bronchoalveolar lavage fluid (BALF) correspond to the inflammatory activity of the hypersensitivity pneumonitis (HP). We studied 12 patients with HP and 13 normal subjects. IL-12 and IL-18 levels were measured using ELISA kits. We found a significantly higher plasma angiotensin-converting enzyme (ACE) concentration (55 vs. 34 U/L, P=0.0016), lymphocyte percentage (57 vs. 14%, P<0.001), CD8+ cells (32 vs. 17%, P<0.001) and a lower CD4/CD8 ratio (1.2 vs. 2.0, P<0.0001). The IL-12 and IL-18 levels in BALF were significantly higher in HP patients than in healthy subjects (3.9 vs. 3.2 pg/ml, P=0.003 and 14.2 vs. 6.15 pg/ml, P% 0.0001, respectively). We found a strong positive correlation between IL-12 and the percentage of lymphocytes (r=0.68, P=0.015) and a negative one between IL-12 and the percentage of macrophages in BALF (r=-0.64, P=0.024). We conclude that upregulation of the Th1 cell cytokine profile may play a significant role in the pathogenesis of HP.