STUDY OBJECTIVE: We compare pain relief and safety of morphine 0.10 mg/kg with 0.15 mg/kg in adult emergency department (ED) patients with acute pain. METHODS: This was a randomized double-blind placebo-controlled trial of intravenous morphine 0.10 mg/kg versus 0.15 mg/kg, (delivered in 2 divided doses) in adult ED patients with acute pain requiring opioid analgesia. Assessment was made at baseline, 30 minutes, and 60 minutes with a validated verbal numeric rating scale. Pain reduction and satisfaction scores were measured at 30 and 60 minutes. The primary outcome measure was the between-group difference in mean before-after change in numeric rating scale from baseline to 60 minutes. RESULTS: Two hundred eighty patients were enrolled. Between-group difference in numeric rating scale improvement from baseline to 60 minutes was 0.8 (95% confidence interval 0.1 to 1.5), favoring the 0.15 mg/kg group. Pain relief scores and adverse events were similar in the 2 groups. CONCLUSION: Although 0.15 mg/kg of morphine is safe and provides statistically superior analgesia compared with a dose of morphine at 0.10 mg/kg, this difference in pain reduction did not reach the threshold of greater than 1.3 numeric rating scale units required to declare the higher dose of morphine clinically superior.