Objective: To determine the effect of high glucose levels on the expression of type I collagen, and whether this effect is regulated by transforming growth factor (TGF)-β1 in human peritoneal fibroblasts in culture. Design: Prospective experimental study. Setting: University medical center. Patient(s): Primary cultures of fibroblasts established from peritoneal tissues of five patients. Intervention(s): High glucose treatment of the primary cultured fibroblasts. Main Outcome Measure(s): Primary cultures of human peritoneal fibroblasts were incubated with varying amounts of glucose (1-5 g/L) for 24 hours. Total RNA was extracted from human peritoneal fibroblasts and converted to cDNA by reverse transcriptase. Multiplex reverse transcriptase/polymerase chain reaction (PCR) simultaneously co-amplifying β-actin with TGF-β1 or type I collagen mRNAs was used to quantitate type I collagen and TGF-β1 mRNA levels in response to increasing glucose concentrations with and without TGF-β1 antibody treatment. Result(s): There was a significant increase in the mRNA for type I collagen and TGF-β1 in response to increasing glucose concentrations in a dose response-dependent manner. The TGF-β1 antibody treatment resulted in an 83% and 68% decrease in type I collagen and TGF-β1 mRNA levels, respectively. Conclusion(s): Increasing glucose concentrations stimulated type I collagen expression in human peritoneal fibroblasts in culture. A potential mediator for this effect is TGF-β1. These results have implications not only for individuals with diabetes mellitus who may be predisposed to greater postoperative adhesion development, but also for individuals with surgical stress responses after surgery.