This prospective, blinded study was designed to assess intraperitoneal cytokine productions and their relationship to clinical presentations and systemic inflammatory markers in patients with inflammatory bowel disease. Fifty patients who required abdominal surgery for active inflammatory bowel disease (ulcerative colitis, 27; and Crohn's disease, 23) were investigated. Interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha levels in intraperitoneal fluid (obtained by intraoperative lavage) and plasma were measured by enzyme-linked immunosorbent assay. To compare intraperitoneal cytokine productions between diseases with a different degree of inflammation, intraperitoneal cytokine measurement also was performed for patients who required surgery for colorectal cancer (n = 25) and acute appendicitis (n = 25). The median intraperitoneal cytokine (interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha) levels were significantly higher in patients with inflammatory bowel disease than in patients with colorectal cancer and acute appendicitis. In patients with inflammatory bowel disease, intraperitoneal cytokine levels were significantly higher in patients with intraperitoneal sepsis (abscess/fistula) than in patients without intraperitoneal sepsis at laparotomy. Intraperitoneal cytokine levels did not correlate with type of disease (ulcerative colitis/Crohn's disease), age, gender, duration of disease before surgery, preoperative medical treatment, and the extent or site of disease. There were no significant correlations between intraperitoneal cytokine levels and the following systemic inflammatory markers: plasma cytokine levels, white blood cell count, platelet count, erythrocyte sedimentation rate, and C-reactive protein levels. Postoperative intraperitoneal septic complications (anastomotic leak/abscess/enterocutaneous fistula) more frequently occurred in patients with higher intraperitoneal cytokine levels. Intraperitoneal cytokine productions were greatly elevated in patients with inflammatory bowel disease, and their levels correlated with the presence of intraperitoneal sepsis at laparotomy and development of postoperative intraperitoneal septic complications. Intraperitoneal cytokine levels showed no correlations with systemic inflammatory markers.