This study aimed to examine the physiological role of hepatocyte growth factor (HGF) after thoracic irradiation. We analysed the changes of HGF protein levels in rat lung following 12 Gy of whole thoracic irradiation. Bronchoalveolar lavage fluid (BALF) was then collected from 11 patients (10 lung cancer and one oesophageal cancer) after completion of radiation therapy. One month after irradiation, the HGF protein level in the lungs of irradiated rats decreased (p<0.05), followed by a remarkable elevation in HGF protein levels 2 (p<0.05) and 3 months (nonsignificant) after irradiation accompanied by the clinical appearance of radiation pneumonitis. Finally, HGF protein levels in the lung returned to their original level 6 months after thoracic irradiation. In humans, HGF protein levels in the BALF in the limited irradiated area were lower than those obtained from unirradiated areas (p<0.05). In conclusion, hepatocyte growth factor production is transiently suppressed in the irradiated area after irradiation.