Objective: To evaluate the effects of halofuginone, a specific inhibitor of collagen type I synthesis, on the postoperative formation of abdominal adhesions in rats. Summary Background Data: Postoperative adhesions remain the leading cause of small bowel obstruction in the Western world. Surgical trauma causes the release of a serosanguineous exudate that forms a fibrinous bridge between two organs. This becomes ingrown with fibroblasts, and subsequent collagen deposition leads to the formation of a permanent adhesion. Most of the drugs used have been clinically ineffective, and none has been specific to a particular extracellular matrix molecule. Therefore, there are serious concerns about the toxic consequences of interfering with the biosynthesis of other collagens, other matrix proteins, or vital collagen-like molecules. Methods: Adhesions were induced by scraping the cecum until capillary bleeding occurred. The adhesions were scored 21 days later. Halofuginone was either injected intraperitoneally (1 μg/25 g body weight) every day, starting on the day of operation, or added orally at concentrations of 5 or 10 mg/kg, starting 4 days before the operation. Collagen α1 (I) gene expression was evaluated by in situ hybridization, total collagen was estimated by Sirius red staining, and collagen type III was detected by immunohistochemistry. Results: The adhesions formed between the intestinal walls were composed of collagen and were populated with cells expressing the collagen α1 (I) gene. Regardless of the administration procedure, halofuginone significantly reduced the number and severity of the adhesions. Halofuginone prevented the increase in collagen α1 (I) gene expression observed in the operated rats, thus reducing collagen content to the control level. In fibroblasts derived from abdominal adhesions, halofuginone induced dose-dependent inhibition of collagen α1 (I) gene expression and collagen synthesis. Collagen type III levels were not altered by adhesion induction or by halofuginone treatment. Conclusions: Upregulation of collagen synthesis appears to have a critical role in the pathophysiology of postoperative adhesions. Halofuginone, an inhibitor of collagen type I synthesis, could be used as an important tool in understanding the role of collagen in adhesion formation, and it may become a novel and promising antifibrotic agent for preventing postoperative adhesion formation.