Background: Acute appendicitis is the most common abdominal surgical emergency especially in children and young adults. The diagnosis of appendicitis is difficult because half the cases are incorrectly identified. Serotonin was defined as a good diagnostic marker for many inflammations including appendicitis and it is metabolite into 5-hydroxyindolacetic acid (5-HIAA) to be excreted in urine. 5-HIAA is suggested to be of diagnostic importance in the detection of this disease. The aim of this study was to evaluate the diagnostic importance of urinary-5-HIAA as an added parameter to Alvarado score. Methods: Seventy patients (35 females and 35 males) with acute appendicitis (35 were mild and 35 severe-perforated and gangrenous) were included in this study and 70 healthy individuals were taken as a control group. Urinary-5-HIAA was estimated in all patients and control group using ELISA method. Results: Sensitivity for the mild group is 94.2%, specificity 100% and diagnostic accuracy is 97.4%, while the sensitivity for the severe group is 37%. It was found that there is a highly significant difference between mild and control groups (P<0.05). The diagnostic accuracy for the mild group is 97.4% and for the severe is 68.5%. Conclusion: We conclude that urinary-5-HIAA is a high sensitive test for early detection of acute appendicitis.