Acute abdominal symptoms in 162 children were investigated with rapidly quantified C-reactive protein (CRP), white blood cell (WBC) count and erythrocyte sedimentation rate (ESR) as laboratory parameters. When the symptoms had been present for at least 12 hours, the CRP level was greater than 10 mg/l in 72% of cases with histologic evidence of appendicitis, while leukocytosis (WBC greater than 15 000/mm3) and raised ESR (greater than 20 mm/l h) were found in 58% and 51%, respectively. If appendicitis had progressed to gangrenous stage, however, CRP alone was capable of identifying 83% of cases, while 76% had leukocytosis and 60% elevated ESR. When perforation had occurred there was greatly increased CRP and invariably leukocytosis, but elevated ESR in only 60%. With the combination CRP and WBC count (using rise in one or both), 88% of all appendicitis cases could be identified, and at least 96% of those with gangrene or perforation. The predictive value of combined positive CRP and WBC tests was not less than 93%.