The aim of this study was to assess the diagnostic value of procalcitonin (PCT) in 212 children with appendicitis and compare it with that of the standard diagnostic modalities, C-reactive protein (CRP) level, leukocyte count, and abdominal ultrasound findings, in relation to the surgical and histological findings of the appendix. A PCT value of >0.5 ng/ml was found to be indicative of perforation or gangrene with 73.4% sensitivity and 94.6% specificity, a CRP level of >50 mg/l and a leukocyte count of >10(4)/mm3 were useful diagnostic markers for perforation, while abdominal ultrasonography had a sensitivity of 82.8% and a specificity of 91.2% for detecting appendicitis with imaging findings. PCT measurement seems to be a useful adjunctive tool for diagnosing acute necrotizing appendicitis or perforation, and surgical exploration will probably be required in patients with PCT values >0.5 ng/ml.