Objectives: To determine the effects of Meperidine (Pethedine®) on pain intensity, clinical findings, final diagnosis and management of patients with acute abdominal pain.Methods: This was a randomized clinical trial including 100 patients, with lower abdominal pain lasting for less than 48 hours who were referred to the emergency department of Imam Khomeini hospital affiliated with Ilam University of Medical Sciences, over a period of 11 months. Hemodynamically unstable patients were not included in the study. The baseline pain severity was measured using a visual analogue scale (VAS). Patients were randomly assigned to receive 25 mg of intramuscular Meperidine (Pethedine®) (n=50) or 5 mL of intravenous normal saline as placebo intravenously (n=50). After 1-hour the patients were then re-examined and the pain severity was re-assessed and the clinical diagnosis was recorded.Results: There was no significant difference between two study groups regarding the baseline characteristics. The mean pain score on arrival was comparable between groups (6.80 ± 1.6 vs. 6.81 ± 1.2; p=0.956). The abdominal tenderness was not affected in Meperidine group. Rebound tenderness disappeared in 4% of the Meperidine group and in 2% of the placebo group. Nausea was decreased in 14% of the Meperidine group and 32% of the placebo group. Changes in the clinical pattern and diagnostic peritoneal signs in patients were negligible and did not significantly interfere with the diagnosis (p=0.133). Diagnostic accuracy was 96% in the Meperidine group and 98% in placebo group, which was not significantly different (p=0.554).Conclusion: Administration of Meperidine reduces pain intensity in patients with acute abdominal pain without interference with the clinical diagnosis. Thus analgesics could be safely administered to the patients with acute abdominal pain for increasing the patients comfort.