Transforming growth factor-beta1 (TGF-beta1) is a growth factor that is implicated in fibrosis of many organs. The purpose of this study was to determine the sequential levels of TGF-beta1 in the pleural fluid of rabbits that had undergone empyema induction, as fibrosis of the pleural space develops. Thirty-seven rabbits underwent empyema induction. Rabbits were sacrificed on Days 1, 2, 3, 4, 5, 6, and 8. Pleural fluid and viscera pleura specimens were collected at autopsy. TGF-beta1 levels were measured in pleural fluid using a commercially available ELISA kit, and pathologic specimens were scored for evidence of fibrosis (pleural thickness and number of fibroblasts). The median levels of pleural fluid TGF-beta1 increased from 8,100 pg/ml (Days 1 and 2) to 39,600 pg/ml (Day 8). Pleural fluid TGF-beta1 levels closely correlated with microscopic pleural thickness (r = 0.7, p < 0.001) and number of fibroblasts present in the visceral pleura (r = 0.68, p < 0.001). The first increase in pleural fluid levels of TGF-beta1 (Day 3) occurred before the increase in pleural thickness (Day 4) and before the increase in number of fibroblasts (Day 4). In conclusion, pleural fluid levels of TGF-beta1 rise in experimental empyema as pleural fibrosis develops. The rise in empyemic pleural fluid TGF-beta1 levels correlates with markers of pleural space fibrosis.