BACKGROUND: Acute appendicitis is the most common surgical abdominal disease in the emergency room. Although the diagnosis is clinical the complementary tests may be useful in doubt. AIM: To evaluate the main laboratory tests in patients with acute appendicitis, as well as its relationship with the evolutionary stage of the disease. METHODS: Prospective evaluation of patients with acute appendicitis who underwent surgical treatment. RESULTS: A total of 179 patients participated in this study, most were male. The mean age was 26 years. For leukocyte count 46.9% had values ​​<15.000mm3. The mean percentage of polymorphonuclear cells was 81,7%, 1,2% of sticks, 1% eosinophils, lymphocytes 12,8% and 2,9% monocytes. C-reactive protein was required for 54 patients. It was <10 mg/dl in 19, between 10 and 50 mg/dl in 24 and greater than or equal to 50 mg/dl in 11. Regarding the evolutionary phase 64% patients had early stage (stages 1 and 2), 16,2% stage 3 and 35 stage 4. A total of 57% of patients with white blood cell count greater than or equal to 20.000/mm3 had appendicular perforation (p<0,05). The percentage of polymorphonuclear leukocytes from patients with early stages was lower than the later stages (79,8% and 85,1%, respectively), with p<0,05. Patients with advanced stages of acute appendicitis, the number of lymphocytes was lower than the initial stages (9,3% and 14,8%, respectively), with p<0,05. 94% of patients with C-reactive protein values ​​<10 mg/dl showed early stages of appendicular inflammation (p<0,05). CONCLUSION: A significant association among total and differential leukocyte count, C-reactive protein values ​​and evolutionary phase of appendiceal inflammation was found in this prospective analysis.