Hematological and biochemical tests, including white blood cell count (WBC), C-reactive protein (CRP) and other acute-phase reactants, have been used in the diagnosis of acute appendicitis. However, there is controversy among physicians about the value of this practice in children. The objective of our study was to evaluate serum amyloid A protein (SAA) levels in children with confirmed acute appendicitis and to compare the sensitivity and specificity of this marker of inflammation with those for WBC and CRP. A prospective cohort study of 60 children admitted with abdominal pain to rule out appendicitis was used in the study. Of these, 42 underwent surgery, while 18 children who had spontaneous amelioration within 24 h of admission were not operated on and served as controls. WBC and serum SAA and CRP levels were obtained preoperatively. Serum concentrations of the analytes were determined with particle-enhanced immunonephelometric methods. Patients with acute appendicitis had WBC, SAA and CRP levels higher than those of the control group (p<0.001). There was no appendicitis patient with a normal SAA value, while 21.4% of the patients had CRP values within the normal range. The performance of each test was measured by receiver-operating characteristic curves. Area under the curve (AUC) values were 0.849 for WBC, 0.868 for CRP and 0.964 for SAA. The sensitivity and specificity of these methods were 76% and 75% for WBC>10.0 x 10(9) /L, 62% and 94% for CRP>10 mg/L and 86% and 83% for SAA >45.0 mg/L, respectively. Circulating SAA levels have better discriminatory value than WBC or CRP in the assessment of acute appendicitis in children. Thus, this test appears to be of higher value than the current standards of care in the diagnosis of this condition.