Background: Laparoscopic surgery involves the establishment of a pneumoperitoneum,mostly using carbon dioxide. Cooling of the peritoneum, due to insufflation, may traumatize the peritoneum and disturb local biological processes. The current study was performed to assess the effect of the temperature of carbon dioxide on peritoneal transforming growth factor β1 (TGF-β1) expression. Design: Patients were randomized into 2 groups. In one group, a pneumoperitoneum was created with carbon dioxide at room temperature; in the other, with carbon dioxide at body temperature. Peritoneal biopsy specimens were taken at the start and end of surgery. Setting: Community hospital. Patients: Thirty patients scheduled for laparoscopic cholecystectomy. Main Outcome Measures: Tissue concentrations of total and active TGF-β1 were measured using enzyme-linked immunosorbent assays. Results: At the start of surgery, there were no significant differences between groups in the total and active fractions of TGF-β1. At the end of the procedure, the peritoneal active TGF-β1 concentrations were significantly lower (P=.03) in patients receiving carbon dioxide at body temperature. In contrast, the concentrations of total TGF-β1 did not differ between groups. A slight, nonsignificant increase in total and active TGF-β1 levels was observed in patients receiving unheated carbon dioxide. The ratio of active to total TGF-β1 did not change during procedures, and there were no differences between groups. Conclusions: Heating of carbon dioxide, used for insufflation, to body temperature decreases the expression of active TGF-β1 in the peritoneum. Considering the broad biological effects of TGF-β1, including the regulation of peritoneal healing and oncological processes, this observation might have clinical repercussions.