This study was conducted to define the value of procalcitonin (PCT) levels in the differential diagnosis of abdominal familial Mediterranean fever (FMF) attacks from acute appendicitis. From October 2006 to January 2007, 28 FMF (12 males, 16 females) patients with acute abdominal attacks and 34 patients (18 males) with acute abdomen who underwent operation with the clinical diagnosis of acute appendicitis were consecutively enrolled in this study. FMF patients with concurrent infectious diseases were excluded. PCT values were measured by an immunofluorescent method using the B.R.A.H.M.S. PCT kit (B.R.A.H.M.S. Diagnostica, Berlin, Germany). Erythrocyte sedimentation rate (ESR), C-reactive proteins (CRP) and leucocyte levels were also noted. Mean disease duration in FMF patients was 9.6 +/- 8.1 years (range 2-33 years) and all were on colchicine therapy with a mean colchicine dosage of 1.2 +/- 0.4 mg/day. Among the operated patients, 5 were excluded: 3 patients had normal findings and 2 had intestinal perforation (PCT levels were 2.69 and 4.93 ng/ml, respectively) at operative and pathologic evaluation. There were no significant differences between the two groups with respect to gender and age (p was not significant (NS) for all). Acute phase reactants and PCT levels were increased in patients with FMF compared to patients with acute appendicitis (0.529[0.12 +/- 0.96] vs 0.095 [0.01-0.80] p < 0.001, respectively). PCT levels higher than 0.5 ng/ml were found in 11% (3/28) of FMF patients compared to 62% (18/29) of acute appendicitis patients (p < 0.001). Our results suggest that PCT could be a useful test in the differentiation of abdominal FMF attacks from acute appendicitis, though it should not supplant more conventional investigations.