INTRODUCTION: Opioid overdose is commonly treated by prehospital emergency services and the majority of the patients are discharged immediately after treatment and a short observation period. There is a minor risk for rebound opioid toxicity and other life-threatening conditions might occur after such episodes. The authors describe the short-term outcome and identify risk factors for death within 48 h after prehospital treatment of opioid overdose in Copenhagen, the capital of Denmark. METHODS: Data on all cases of opioid overdose treated by the medical emergency care unit between 1994 and 2003 were recorded prospectively. Risk factors for death within 48 h after initial medical emergency care unit contact were analysed in a multivariable logistic regression analysis. RESULTS: The authors recorded 4762 episodes of opioid overdose, covering 1967 unique identified patients. A total of 78 patients (8.4%, 95% CI 7.0 to 10.4) died within 48 h in the period 1999-2003, and 85% (66/78) of these had cardiac arrest and died. The authors found age >50 years and overdose during the weekend significantly associated with 48-h mortality. Gender, former episodes of opioid overdose, time of the day, month or year were not significantly associated with increased mortality. CONCLUSIONS: The author found a 48-hours mortality of 8.4%. Advanced age and opioid overdose in the weekends were significant risk factors. Release on scene after treatment was associated with a very small risk.