Serum C-reactive protein was measured in 56 patients hospitalized with a suspected diagnosis of acute appendicitis. Based on these determinations, four groups of patients were defined: Group A=26 patients with acute appendicitis who had a C-reactive protein level higher than 2.5 mg/dl. Group B=4 patients with a C-reactive protein level lower than 2.5 mg/dl who, after surgery based on a presumed diagnosis of acute appendicitis, were found to have a normal appendix. Group C=22 patients with nonspecific abdominal pain, 18 (72 percent) of whom had an elevated C-reactive protein level, although in only 4 (7.1 percent) were these levels higher than 2.5 percent mg/dl. Group D=4 patients who had diseases other than acute appendicitis. It is concluded that an increase in C-reactive protein levels to more than 2.5 mg/dl is not a definite indicator of acute appendicitis. However, if the C-reactive protein level in blood drawn 12 hours after the onset of symptoms is less than 2.5 mg/ dl, acute appendicitis can be excluded.