Determining the different cutoff values of C-reactive protein (CRP) on the basis of how long the patient's symptoms were present can be used to early predict acute appendicitis. We analyzed retrospectively from 2001 to 2004 the hospital records of 568 patients who underwent appendectomies for suspected appendicitis. Receiver operating characteristic analysis has shown that CRP measurement can increase the diagnostic accuracy in acute appendicitis. The cutoff values of CRP concentration taken as the first, second, and third days after onset of symptoms that distinguish acute appendicitis from other acute abdominal diseases were 1.5, 4.0, and 10.5 mg/dL, respectively; the values that distinguish perforated appendicitis from other acute abdominal diseases were 3.3 mg/dL (first day), 8.5 mg/dL (second day), and 12.0 mg/dL (third day). The different cutoff values of CRP concentration may serve as a useful predictive parameter in the early diagnosis of acute appendicitis on the first 3 days after the onset of symptoms.