BACKGROUND Preoperative measurement of body inflammatory agents reduces unnecessary appendectomies by up to 30 percent. A decline in the formation of blood leukocytes and C-reactive protein with aging may hinder the correct diagnosis of appendicitis. METHODS White cell count and C-reactive protein were determined before appendectomy in 600 patients aged 0 to 5 years, 6 to 19 years, 20 to 39 years, 40 to 59 years, 60 to 79 years, and older than 80 years. Their records were analyzed. The sensitivity, specificity, diagnostic accuracy, and receiver-operating characteristic curves for C-reactive protein and white cell count to predict appendicitis were calculated separately for each age group. RESULTS The rates of negative explorations and perforations were highest at both extremes of age. In uncomplicated appendicitis, the diagnostic potential of white cell count was better than C-reactive protein in all age groups except infants. The C-reactive protein was elevated similarly throughout human life, but only in those with perforated appendicitis. The receiver-operating characteristic curves confirmed that the performance of white cell count was better than C-reactive protein in the correct diagnosis in every age group except infants and octogenarians. CONCLUSIONS The leukocyte response declines in 0- to 5-year-old children with appendicitis, but the C-reactive protein response is well preserved in all other age groups.