C-reactive protein(CRP), a normal constituent of the serum in healthy individuals, increases in response to various bacterial infections and cellular necrosis. To find out if the C-reactive protein concentration is of any value in the diagnosis of acute appendicitis, ninety consecutive patients suspected to have acute appendicitis were studied prospectively. Of these, 54 patients(group A) had acute appendicitis and 6 patients(group B) underwent surgery with a diagnosis of acute appendicitis; however, histopathology disclosed a normal appendix. Seventeen patients(group C) had no identifiable cause for their illness and 13 patients(group D) had an identifiable cause for their illness, though not appendicitis. The mean serum CRP value was 49.00mg/L in group A, 18.92mg/L in group B, 20.31mg/L in group C, and 63.22mg/L in group D. Within group A, the mean CRP value in the 36 patients with the inflammed appendix was 33.56mg/L. The mean value in the 7 patients with the gangrenous appendix was 73.16mg/L, and 97.61mg/L in 11 patients with the perforative appendix. Serum CRP levels were normal in two patients with acute appendicitis. Besides these 2 patients, all patients with normal CRP had a normal appendix found at the time of operation or their symptoms resolved spontaneously. It is concluded that an increase in CRP levels to more than 5 mg/L is not a definitive indicator of acute appendicitis. However, when there is doubt about the diagnosis of acute appendicitis, a normal serum CRP level should be used as a basis for the decision to defer surgery.