BACKGROUND Acute appendicitis is one of the most common abdominal emergencies. The clinical diagnosis is often difficult even for experienced surgeons, however, as evidenced by the high rate of negative explorations. A delay in diagnosis of acute appendicitis is associated with increased risk of perforation and further complications. The aim of the present study was to assess the preoperative YKL-40 levels on for a clinical suspicion of acute appendicitis. METHODS Between August 2008 and December 2008, a total of 34 patients who underwent appendectomy with a clinical diagnosis of acute appendicitis were studied. Patients underwent appendectomy with the preoperative diagnosis of acute appendicitis. The appendix specimens were classified as normal appendix (group 1; 10 patients), acute appendicitis (group 2; 24 patients). Serum YKL-40 levels were determined by a commercial ELISA. RESULTS The levels of serum YKL-40 were significantly higher in the group 2 compared with the group 1 (66.4 +/- 13.2 vs 41.6 +/- 11.6 ng/mL, p < 0.001). Receiver operating characteristic curves of YKL-40 levels counts was on a statistically significant level (area under the curve [AUC] = 0.926, p < 0.001). CONCLUSIONS YKL-40 may be a useful marker for diagnosis of acute appendicitis but the number of subjects was limited in this study, future studies are required to confirm the results presented here (Fig. 1, Ref. 13).