Hypothesis: Inflammatory markers differ between subjects with appendicitis and controls. Markers of inflammation differ in serum compared with intraperitoneal fluid. Among subjects with appendicitis, inflammatory markers differ between subjects with and without perforation. Design: Cross-sectional. Setting: Hospitalized care. Patients: Twenty-four children who underwent an appendectomy. Group A (n = 19) consisted of patients with appendicitis and group N (n = 5) of patients with normal appendixes. Main Outcome Measures: Serum and peritoneal levels of interleukin (IL)8, IL-10, granulocyte colony-stimulating factor, interferon γ soluble intercellular adhesion molecule-1, matrix metalloproteinase-9, and tissue inhibitor of metalloproteinases-1 were measured by enzyme-linked immunosorbent assay. Results: Age, sex, complete blood count, C-reactive protein level, and serum cytokines did not significantly differ by group. Peritoneal concentrations of interleukin-8 (mean ± SD, 1416.8 ± 1436 pg/mL vs 48 ± 74.4 pg/mL, P = .001), IL-10 (mean ± SD, 3085 ± 5893 pg/mL vs 84 ± 46 pg/mL, P = .02), matrix metalloproteinase-9 (mean ± SD, 1784 ± 1225.1 ng/mL vs 435 ± 563 ng/mL, P = .03), and tissue inhibitor of metalloproteinases-1 (mean ± SD, 8939.2 ± 7312.2 ng/mL vs 602.1 ± 345.6 ng/mL, P<.001) were significantly different in group A compared with group N. When compared by perforation (n = 8 with perforation vs n = 11 without perforation), peritoneal granulocyte colony-stimulating factor levels were elevated in subjects with perforation (mean ± SD, 4.3 ± 14.4 pg/mL vs 62.7 ± 79.2 pg/mL, P = .02). Although serum tissue inhibitor of metalloproteinases-1 was not different between groups N and A, it was significantly different between group N and patients with a perforated appendicitis (mean ± SD, 205.9 ± 43.8 ng/mL vs 3068.9 ± 5122.4 ng/ mL, P = .04). Conclusions: Presently, it is not practical to differentiate appendicitis in a pediatric population from other causes of abdominal pain based on the detection of systemic inflammatory response markers.